Answer 1

The stack based VM use a memory structure where the operands are stored in a stack data structure. They generally use the LIFO to push and POP value to do the operations. This mean that the VM don't really care about the operands addresses because the POP will give the next one. And the register based implementation is the oposite. They are ni PUSH or POP operation. The instruction need to contains the addresses (the registers) of the operands.
Simple exemple of an addition with assembler look alike code:

Stack base:
POP 2 # get value
POP 3 # get Value
ADD 2, 2, result # make the addition
PUSH result # store the value

Register based:
ADD R1, R2, R3 ; # We had the contant of R1 and R2 in R3