I've never had a bad experience on Vinted, as either a buyer or a seller, and I've bought a hell of a lot from there and sold a few items, too.
I accept a lot of things are par for the course with it. Vast majority of people aren't "professionals" using it as a platform to sell their goods-they're just regular people, having a clear out.
If you need something sent by a certain date because you have an event that you need it for-then make the seller aware of that before you buy and ask if they'd be able to send it ASAP (but if you've left it last minute to buy what you need-don't be going to Vinted and crying that it's not going to arrive in time). They have 7 days to send by Vinted rules.
Always check the reviews of the sellers before buying. If the person has form for not sending items in the past- don't be surprised when it happens to you. I don't buy from anyone who has form for that or for sending items not as described.
Understand that people have lives-this isn't Amazon. You cannot expect same-day delivery or one-day delivery.
Don't send lowball offers and expect a response. Some sellers don't even like offers at all so don't be offended if it's not accepted or even declined. Some don't want to get into a bartering session with you.
I won't buy from people who only post one picture of the item and think that's a job well done. If you're only posting one picture, it makes me wonder what you are hiding on the areas of the item you're not showing.
Don't list an item as being X size, and then put in the caption "It's actually a Z size, but fits more like X." I don't care about your opinion of the size-I just want to know what size it was listed as by the store. You can list it as the correct size and then put in the description that you think it's a big/small size Z and would be better for X size-but don't confuse matters by listing it incorrectly. If I'm looking for a UK 10, I don't want UK 12 items to come up.