It's a short ranged bomb and people can dash out of it (and more rarely, striker-boost out of it). That aside, it also makes you walk really slow while charging it. At face value, it doesn't sound like it can work. Heck, after the dash was added to the game, people were like "aw now vortex is useless because people can just dash out of it". But it's exactly the fact that people can dash out of it that makes it have some use: If they dash out, they won't be able to dash for 8? more seconds. If they don't, they're at your mercy inside the bomb.

If anything, vortex is an 'opener' offensive bomb - if you catch someone by surprise by placing it somewhere they're boosting towards, you can get a free hit, if not combo. If it shocks after that, you get a free kill. If someone gets caught in one, you can put one more 3-4 tiles behind it, to catch them after they dash. But after those first (two) bombs, you should switch to another weapon.

For offensive bombs, you don't use them to wall, but to catch people who really want to move towards a certain area. Walling is easy, but it takes a little more intuition to use bombs offensively.

Also, selling it because people were making fun of you in lockdown was bad, because:
1) It has its uses if you place it smartly (robbing dash, getting multiple kills at once, bullying careless strikers... etc).
2) It's amazing and saves you a LOT of time in PVE when it comes to killing 6+ monsters.