It's good that you recognize the need to do accuracy upgrades before power upgrades, so I think you have a good start there. You will find that some of the same mods that improve your accuracy will also increase your FPS as a side effect because they will improve air seal.
I've owned two VSR 10 variants and worked on a couple others, so I know my way around them pretty well, and I can say with confidence that some of the best mods for the VSR platform are DIYs that may even need to be done with aftermarket parts. I would suggest that you try out some of the basic easy DIYs to begin with in order to see what kind of a difference they make. Add them one by one, and then eventually move on to upgrades until you find the performance you are looking to achieve. Here is the basic rundown of what I think is important on the VSR platform.
1. Air seal. On a stock setup there are 3 main leakage points: piston o ring, cylinder head threads, and hop up bucking. If you search for vsr air seal mods you will find the common fixes for all of these points. But to get you started I'd recommend you search for anti blow-by rings, Teflon mods, and new o rings IIRC the piston o ring is size #13.
2. Hop up and barrel. You can try polishing the stock barrel and just upgrading the bucking, or go straight to an upgraded barrel. PDI and Laylax (Prometheus) are widely regarded as the best VSR barrels. A tighter bore barrel will also boost your fps as well. As for buckings, most soft buckings will be a good upgrade over stock. Nineball is one of the preferred ones, but I had good results with just a cheap lonex one.
3. Good bbs. This is constantly being debated but you definitely want something over .30. Goldenball, bioval, and bioshots are all good. There may be some others out there by now that I haven't kept up on though.
3. Power. If you get the airseal right you will find that VSR springs are underrated. I gained about 50 fps on my last VSR 10 clone just by doing airseal mods. If you want to get into the mid to high 400s I'd say go with a 130 spring and a new barrel as well as the obvious airseal mods.
Hopefully this gives you a good start. Back in the day a guy on airsoftretreat by the name of Noobie came up with a bunch of very innovative upgrades/DIYs for the VSR platform so I'd highly recommend you take a look at his stuff. I don't think he is around anymore but all the information should be there.
