Do
you play them? Actually, which games do you mean?
Do you mean games like Space War and Asteroids (first thing I think of when I read 'vector space shooter')? Or
shoot 'em ups? Or 'arena' shooters like Geometry Wars (vectors here too), Robotron, Everyday Shooter?
Shmups are a hardcore niche. But there is a more casual side too; they're common enough on Flash portals. Because they're simple to make. You'll have plenty of competition on both sides...
Arena shooters were common in the console downloadable area a while back.
Everyone learning programming right now is making an Asteroids clone.
Which type of commercialness are you aiming for? (Selling an executable to play offline, ad-supported browser game, or... well those two
seem most suitable)