It is very easy to understand.
DCA is fixed amount investment every month, for example every month you invest $1000.
In VAP, you investment amount is more like a "top up" to match a particular portfolio level each month.
For example your goal is you portfolio should increase everymonth by $1000. If lets say if market doesnt move at all, then DCA = VAP = $1000 every month.
But markets go up and down, let say this month your portfolio value is 10k and next month you target it to be 11k, but market falls a bit so it actually ends at 9900, so you top up 1100 to hit the target level of 11k. But if markets rise a bit and actually ends at 10100, then you only top up 900 to hit the target level of 11k.
So in VAP, you need to have a cash pile that is seperate from your investment portfolio and your cash pile is used to top up the investment portfolio every month to hit the target level, if markets rising very fast, your cash pile will grow a lot, because you top up very less to your portfolio. If markets are falling fast, you spend a lot from your cash pile and the danger is you could run out of cash. So you need to set your portfolio target monthly increase to a reasonable level, not too high not too low.
If you are into driving , an analogy I can think of DCA is like filling your car petrol every week 10Ltr. While VPA is like every week you just do full tank.