Decay is meaningless in the vix ops. You should be looking at the vix term structure or the vol of vol. If i understand your style correctly you are betting on index vols not decay. Which is essentially a long/short index bet. Might as well sell/buy vol outright then mess with the vix ops.
If its positive theta you are after you should be long the convergence via long calendars/diagonals. Good luck.
If its positive theta you are after you should be long the convergence via long calendars/diagonals. Good luck.