I don't know for sure about this, but as it seems to me, the VX boards used to have only 6 (or 3, dunno) RAS lines (Assuming this is the real word, I don't recall the exact word now).
Possibly your DIMM1/SIMM1&2 share the same RAS lines. Then you'd have 3 banks, like:
bank 1: DIMM1 or SIMM1&2
bank 2: DIMM2
bank 3: SIMM3&4
Technically, it *would* be possible for you to fit memories in all slots of the shared bank (DIMM1 and SIMM1&2) *if* they are of the same technology (FPM/EDO/SDRAM), *and* they are single-sided (i.e. use just 1 RAS line instead of 2, so one completes the other).
Otherwise you'll lose one of the SIMM banks.
Again I can be wrong