As previous posters have said, a VSR of some description is one of the easier rifles to upgrade due to the availability of parts. If you can, get a TM as their hop units are brilliant stock.
Upgrades wise it's generally accepted to do accuracy upgrades before power but that's dependent on your budget. There are a lot of DIY mods you can do for pittance if you're short on cash, a few of them here:
1. Barrel polishing - removes any unwanted friction from your barrel.
2. Barrel spacers - stops unnecessary movement of the inner barrel making your rifle accuracy more consistent.
3. Washing BBs - removes grease/residue from the BBs which means the hop rubber grips them better resulting in a better flight.
That said be looking at putting in at least a new tightbore barrel, spring guide (metal), spring and trigger set up as a minimum if you want a more powerful rifle. Maybe even a new hop up if it's a clone rifle.