Part 117 (piston spring) is a usual breaking point, as well as the piston/piston head. The components of the recoil unit have been out of stock for a long time, some people experience issues with the bolt in terms of wear. Magazine feed lips are also a bit of a pain when they break but I believe milspec have a few in, which is very odd. People from the States have been ordering the few Vector parts that Milspec has so it would be best to take a look now rather than later.
I've owned 4 or 5 Vectors over the years, it is recommended that you own at least 2 at once, in case one breaks and needs spares.
Random, but careful how you pull the charging handle. Seen many Vectors where people have gradually bent the charging handle upwards slightly which can cause issues down the line.
Other bits like the magazine baseplates are a common breaking point.
Great guns, but are becoming more of a collector's piece as of recent due to rarity of parts.