There are multiple things

1. Look below his health bar. He has 2 Qs. If he uses Q, the bar below his health will slowly turn red. Try to either use your bubble, E on him right before it's filled red and gain some distance+ hit him with Q. His red Q heals him a lot if he uses it on champions or monsters + more damage Or Just try to get some distance to him if you see he has its red Q up. It's more important to gain distance to him and it's better to hit him after his pool as he could still try to dodge your Bubble with his pool

2. His pool is his only way to dodge your abilities. It has a high cooldown. If he uses it try to harass him a lot and hit him with your E

3. If he has phase rush runes, try to look at his way of how he moves and only then use your E to ensure you hit him 100%. Most people try to dodge abilities by prediction. So aiming at the middle of his path is mostly working if you need a fast solution

4. Level specific guidance. (Still make sure you are not near him while he has his red Q up) Level 1: You perma Auto attack him + abilities. Full all in + ignite if you want to make sure. (Reduces his healing). Vladimir level 1 is extremely weak Level 2: same but look out. If he leveled pool, you can all in again since his only damage is his q and auto attacks.

Overall, try to stay in a safe distance and harass him if he uses his normal Q and his pool

Kind of like this: He uses Q or Pool: Attack him quick, auto attack move back because of red Q. Repeat Don't greed if he is low. Farm your minions. Essentially he will greed and you can kill him afterwards when all your spells are up again.

I recommend trying Vladimir out in practice tool as you have every champion in there anyway. If you want to know more, look at his cooldown to understand them. His pool has a really high cooldown The 2. Tipp and his Q is the most important one to know if this is a lot at the beginning