How acidic does a mixture have to be for milk to curdle? I am planning on making sausages with both cream and vinegar as ingredients. I suspect the mixture of all the ingredients with the vinegar won't be acidic enough to cause the cream to curdle, but this has raised a question: how acidic does a mixture have to be for milk to curdle?

Certainly if I drop milk into 100% vinegar the milk will curdle. But what if only 20% of a mixture is vinegar? 10%? 2%?
Milk's natural pH is about 6.5, just slightly acidic. If it approaches a pH of 5.5, the casein proteins lose their negative charge and the micelles no longer repel each other, meaning they start to gather in small clusters. At around pH 4.6, the scattered proteins bond to each other again and begin to curdle. 

Though this kind of curdling is not the same as when making cheese with rennet.

I don't know what proportions of milk and vinegar you need to keep the milk from curdling but using pH levels you should get a pretty good idea.