HI Stockeiro,
"Nao". What is your country?
I don't know if you speak spanish or portuguese. I can explain in portuguese. Just post all your questions. It can be very complex because of copyright laws.
When we are accepted into Shutterstock, they makes us read the "photographer training manual" where they explain all of this. You can also read the Artist contract, I think it explains what you can and cannot do.
Please post a link to your portfolio this way we can give you better advice.Answering - Most free vectors, are still in copyright to the owner.
Also keep in mind that a loooot of free stuff which is stolen and posted by hackers....
The only thing you can use are public domain images the ones , the ones created before 1920...but not all agencies accept that, because they already have a lot of it. Some still accept.
Most of these public domain images come from antiquarian books, posters, postcards, photos from before 1920, most which fell into public domain; however if you were to use some of those images, you need to scan directly from the old books that you buy.
If I find a free vector, I can use it to supplement my design and resell in microstock?