How to make pie filling which tastes like yogurt icecream?
I'm not much of a pie baker, but I am something of a chemist.  So, I can tell you that the yogurt is pretty sour (pH around 4.4.-4.7), but the acidity comes from lactic acid, which tastes different than the acetic acid of vinegar or the citric acid in citrus fruits.  

I think it's the flavor of citric acid you want, and are missing, not the actual acidity (with cream of tarter in there it should be quite acid).  In this case, lime is a natural partner to papaya. Rather than flavoring the yogurt with vanilla, why not use a healthy jigger of lime juice and its zest?  The juice has a pH of around 2, so it will also increase the acidity some.  The zest gives the essential oils of the fruit, giving a stronger lime smell and flavor (without actually impacting the acidity).