inb4 hots is a dead game meme
I've never played Vainglory so I can't say which is better. I have, however, played copious amounts of Hots and continue to play with friends. In spite of the team's downsizing, the skeleton crew of devs are still very devoted to the game and continue to roll out patches every few months. Recently, the game received a new update reworking a hero and the tower targeting system to be more like contemporary mobas.
Heroes of the Storm forgoes the popular item-purchase to scale damage in favor of a talent system. This allows players the capacity to customize and play several builds on one character alone thanks to talent synergy. The downside with the talent system is that, at the end of the day, some talents are better than others. Against certain team compositions, you are shoehorned into picking a particular talent to counter, but I think it lends to its novel playstyle that other mobas can't replicate. E.g a Caitlyn in League will always build AD and go auto attack. Meanwhile, a Valla can build Hungering Arrow, Multishot, Glorified Range Minion, or misc.
There are many maps, each with their own unique objective like a king of the hill or boss fight. Winning the objective often grants an temporary edge in besieging the enemy towers. This offers counterplay, such as focusing another objective like split pushing or taking mercenary camps (Hots version of jungle), defeating the camp temporarily bribes them to your team to fight in your lanes.
Consequently, this means there are less opportunities for soloplay/carry due to its team-oriented objective focus. Players and critics cite this as one of Hots biggest weakness- individual players rarely find glory in singlehandedly winning the game like in League or DOTA where there is a designated carry.
On the other hand, given its team focused combat you can create the biggest wombo combos most other mobas can only dream of. This can lead to some satisfying plays when pulled off right, as well as very disappointing plays if you fail. It also means that you may not have as much fun in soloq. I'm lucky to have friends who also enjoy Hots, and we're able to communicate to avoid the more frustrating aspects of the game.