Coincidentally enough I did the very same thing over the weekend. I'd made a vindaloo and just happened to have some potatoes I'd prepared for a Bombay Aloo, using the method from CBM's book, so thought "Hmm... those will do nicely".
And they did.
But I did detect a slight clash of flavours betyween the vindaloo sauce and the potatoes, which I put down to the presence of fenugreek and fennel seeds in the panch phoran (bitter and aniseed respectively).
I wouldn't call it a failure, but that said I probably won't do it again. Essentially, you're mixing together two very distinct and individually balanced dishes that are meant to be enjoyed as such. It wouldn't matter if the cuisine was French or Italian - if you were to add something from a side dish to the main, it would alter it completely.
Of course, there's always the possibility of chancing upon an improvement. After all, without experimenting we wouldn't have some of the great dishes that exist today.
But I'll be sticking to my usual method for cooking potatoes for vindaloo, which is in salted water with a little turmeric for colour.