It depends on the blog. You can pay to sponsor, and then they may include you in a giveaway or a post to promote your product. Some bloggers accept gifts from shops and if they like the item, then they may post about it -but usually don't make guarantees.
I've seen a couple vintage shops on some popular fashion blogs that usually sponsor the blog, and then do a giveaway for a gift certificate to the shop. It definitely would be great promotion.
I've done two blog giveaways before, and both brought in sales, and one a repeat customer. It's definitely something to try and see how it works for you. Make sure to really research the blog beforehand, and find one that has had successful vintage giveaways before if you're thinking about doing it.