1. If I use generated content from VecMaker on products like T-shirts, prints, etc. for example, am I able to sell them (including pre-existing content)?
2. If I buy VecMaker, is do I get life-time upgrades without paying more?
1.- Yes you are able to sell in personal/commercial projects, tshirts or whatever you do with Vecmaker, even with existing content like shapes in galleries or whatever.
2.-Yep, after buy is lifetime upgrade free. Today i updated to 1.01 due some bugs on clone and multiple selection. All my tools/assets on itchio are pay once and free updates lifetime.