- Tranquil Boots
If damage is taken (20 minimum) 4 times in 10 seconds, Boots become Boots of Speed until the last 10 seconds don't have 4 instances of damage.
70% chance to block damage (40 if you are Melee, 20 if you are Ranged)
+4 HP / sec to all allies in 500 radius
These boots are amongst the best items in the game right now. Cheap cost, paired with an excellent heal for a low mana cost make them a great option for a number of heroes. Being able to disassemble them later on is even more of an incentive to buy them.
You'll frequently see these picked up on off-laners who need the HP regen to live through the trilane harass. Carries also pick these boots up frequently for the burst of healing and the low cost gives them a decent set of boots that allow them to hit bigger items faster. Junglers also pick these up frequently (specifically melee junglers).
Burst Healing vs. Sustained Healing
In a lot of ways, the burst of healing is much better than a sustained healing (e.g. Vanguard). In a high level setting, you're more likely to see a coordinated attempt to quickly burst a hero down (e.g. double stun) than a spell here and there. If you live through the combo, you can retreat and be back in the fray within 10 seconds.
On the other hand, an item like Vanguard would take significantly longer to allow you to return to the action.
Downsides and CopingThere are some downsides to Tranquil Boots. You don't get any HP, they are terrible for escaping (being able to break), and they give no stats.
But like every item, the downsides aren't so bad if you realize how to play around them. You want to be the aggressive side of a conflict if you get these, static farming, or already have a good escape mechanism.
OverviewAt the moment, Vanguard is fairly unpopular at the pro level. In many ways, it's overshadowed by Tranquil Boots. The two items fill a similar role, but Tranquil Boots are a fraction of the cost of Vanguard, making Tranquils much more appealing. Having an extra 1250g means you're able to get big items faster.
That said, Vanguard is still a very powerful early-mid game item if you can farm it fast enough, but really only if you're a melee hero. The HP and Regen are great, but the damage block is devastating early game. On average, you're blocking 28 damage per auto attack, which is roughly half of many heroes base damage. However, you must take into account that much of the damage early game is from spells, but that's where the 250 raw HP is useful.
Downsides and CopingThe real downside of Vanguard is the price, and to some extent the rate of regeneration.
To make the most use of the item, you'd want to get aggressive early on and be involved in many team fights. Early game is where the damage block will make the most difference (unless you plan on tanking creep damage).
If you plan on getting a farming item (e.g. Hand of Midas, Battlefury), then you will not want to pick up a Vanguard. It will set you back too much.
If you plan on farming heroes, this might be a good option. However, items like BKB may be more important to pick up early
If you are an Illusion hero, you're better off picking up the Vitality Booster and saving it for a Heart. As far as I know, your Illusions only benefit from flat HP, stats, and certain attack modifiers (e.g. Diffusal Blade). Damage Block animations may play for your Illusions, but the damage is not blocked.
At best, Vanguard is currently a luxury item similar to Bloodstone. It's nice to have, but no one absolutely needs it. The price is too high for what you get. If you get it, that should only be if you've been fed several kills very early on and plan to keep playing aggressively.
OverviewMek is generally considered a "support" item, but in reality any hero can pick it up (e.g. Carries like Necrolyte frequently pick Mek up). A burst heal is good at any point in the game. The item gives a lot of good stats and the HP regen is very good, for being an aura. The components are small (other than the recipe), so it's quite easy to build up to.
Some pros have argued that Mekansm is more efficient than Vanguard and that if you're considering Vanguard, to get a Mek instead. A burst heal for the entire team can turn a fight around moreso than a single hero with Vanguard.
Downsides and CopingThe main issues with Mek are that it is expensive (though made up of inexpensive items), it requires activation (which can be hard to remember in the midst of a fight, especially if you've taken the least damage), and the mana cost is high (150). The heal and armor do become less helpful late game, but 250 burst heal is still decent.
To cope, you can build the item over time. It's generally more beneficial to get Buckler early because it has the active armor bonus that can essentially be activated all the time (cooldown and duration are the same). It's great for pushing towers as well.
During a fight, you just need to make sure you don't get initiated on so that you can activate Mekansm.
Lastly, it helps to be a hero with a high mana pool so that you can afford to use Mekansm often and when needed.
Final ThoughtsAll three items are great for early game sustainability.
Over the years, Vanguard has fallen a lot in popularity. Changes that nerfed it ranged heroes started the downfall. With the introduction of Tranquil Boots and their rise in popularity, Vanguard has been overshadowed.
Another big factor seems to be a pacing change in the game. Vanguard was picked up considerably more when popular strategies did not involve as much early game pressure. Carries don't have the time to farm up a Vanguard just to be safe.
I would not go so far as to say Vanguard is a "bad item", but there are too many other items that are higher priority, and a number of heroes that can mitigate the downside of not having a Vanguard. For instance, Magnus is a powerful initiator with a long duration stun. Having several seconds to attack your opponents care free means you can prioritize damage items over something that is more for survivability.
As for the other items - Mekansm has always been popular. Given that Vanguard only blocks 20 damage on Ranged heroes, Mek is a pretty good substitute if you really want that extra survivability. It's an item that doesn't always have to be pawned off to the supports. In some cases, it can be hugely beneficial for a carry to get it. For example, Necrolyte is frequently in the middle of the fight so that his Pulses hit everyone. Having him carry a Mek is convenient for healing your entire team with ease (as opposed to a hero like Sniper who is off to the side). Razor is another good candidate, though lacking in mana.
Tranquil Boots are border line broken at the moment. They provide so many benefits like burst healing, cost effective, cheap, disassemble-able, and a lot of speed. If you're on the offensive, they aren't very likely to break, which makes them great for aggressive carries that want the heal and speed, but don't want to put a lot of money into boots early game.
In my opinion, Tranquil Boots have completely obsoleted Vanguard, and Vanguard is in need of buffs or is only going to be seen as a niche/luxury pick up going forward.