For the Bi-Pod, it should mount directly to your VSR-10, I am pretty sure they have the standard Bi-Pod Swivel Mount. So, all you will need is the Bi-Pod.
AE's are supposed to be the best (If I remember right, Blacks are the best) for .20s - .25s (which you should run with that spring as it will only put you up to around 400fps).
I usually use AEs when I can, Excels when I can't get my hands on any AEs in time for standard AEGs. For my PSG-1 and my higher hitting rifles, I run at least .29s (Maruzen S.G.M.'s).
What FPS are you wanting to achieve? A PDI 120% comes out to around 400fps right? That's sort of weak for a rifle you are going to be limited to bolt action with. Sure, you may be able to get away with just upgrading the spring for a while with nothing else breaking, but, if I were you, I would go for something a bit
heavier. Say for example a Laylax PSS10 PS150
(almost 500fps, around 490). (Will require you to upgrade parts along with the spring though).
Then, upgrade to at least .30 bbs (you maybe able to get away with Maruzen Super Grand Masters, but I would try and run .33s if you can
or go with KSC .30s
Also, heres a list of parts that should go with each FPS upgrade:
Dusk's VSR-10 Upgrade Guide