It sounds like you're researching outside your vehicles' effective range.
At the end of a match, the same number of RP is basically awarded three times: As modification research, as vehicle research, and as convertible research. These three are independent of each other, RP is not shifted around between them. If you got 1500 mod RP, you also got 1500 vehicle RP.

However, there are penalties if the vehicle(s) you play and the vehicle you're researching are more than one rank apart. You're only getting ~1/3 of the vehicle RP awarded, so I assume you've got a two-rank difference. The wiki has a full table for research efficiency, but basically, to actually get the full amount of RP you earned applied to a vehicle, don't research more than one rank above or below the vehicles you're playing.