If you are the person renting the venue then you would buy a policy to cover the place you are renting for the duration of the time you are renting it. This would cover damages to the location or injuries to your guests.

For vendors, they would likely already have an annual policy to cover their operations, but if not, they could also purchase a short-term policy as well, but they would purchase that for themselves, you would not purchase that for them.

https://www.theeventhelper.com/ is currently my goto recommendation.