Well the moment your gain pts cross 1000 pts earned through txns you get this per lakh benefit.
If you use physical DC you earn 1X Normal pts only. So for that you need to do 1L txns but with VDC you get extra pts that complete your milestine at half so 50K on VDC gives 500 Normal + 500 Bonus = 1000 Pts. Now whenever you earn 1000 pts it is equal to 1L and milestone triggers. This is a bug which may or may not get rectified by SBI.
Just because of this you also get accerlerated given for both Normal and Bonus during offer campaigns.
Physical DC = 50K Txn = 500 Normal Pts. So no milestone until you spend 1L with it.
VDC: 50K Txn = 500 Normal 1X + 500 Bonus 1X = 1000 Pts that trigger and tell system spend of 1L completed. So in short, when you pay or cumulatively reach 50K, you get this 1000 also turning your all txns into 1% cashback. If this month you do 25K only and next month 25K even thern also you get milestone benefit.
But if you miss bonus pts credit then you lose this benefit also.
Ex: You did 50K txn and no bonus credited then you need to reach SBI Rewardz CC and they will credit it manually but then this will not be counted in miestone which trigger automatically. So 1% these days isn't guaranteed.
Now another ex: You did 50K and missed bonus, got only 500 Normal and later you did 25K and got normal + bonus also then 500+250+250 will trigger 1L milestone benefit.
In short whenever your earned total pts incl. normal+bonus reach 1000, you get it. Accelerated RPs or compaign bonuses aren't part of 1L milestone.
