Yes, you can often do this substitution and I've done it successfully many times when I've run out of milk unexpectedly, but you should know that the substitution is never going to be perfect.
On a purely mathematical basis, let's say your heavy cream is 36% M.F. You need partially-skimmed 2% milk for your recipe. If you go purely by weight (which is almost 1:1 for volume when talking about mostly water), then 250 mL of 2% milk will contain about 5 g of milk fat. To get that from 36% cream, you only need about 14 mL, or approximately 3 teaspoons of cream. (Note - don't do this, read on!)
Of course, diluting cream at a ratio of 15:1 is ridiculous and you'll end up with something that's just really watery, and that's because milk is more than just water and fat. I only provide the above calculation for illustrative purposes, to show why there's no hard rule or even rule of thumb for this particular substitution.
With normal cream, most people go with half water, half cream, or if they're trying to approximate skim milk then maybe 1 part cream to 2 parts water. If you have double cream, I would probably use 1 part cream to 3 or 4 parts water; that will give you a reasonable approximation of the consistency of whole or skim milk without making it too watery.
Please keep in mind that different recipes/preparations use milk for different reasons, so this isn't going to work everywhere. I certainly wouldn't do it in baking, but for many stovetop recipes, the main purpose of the milk is just moisture, so you're fine to substitute cream + water and it doesn't really matter if you're "exact" - which you can't be anyway.
Actually I would disagree to some extent with the other two answers - I think you can use creme fraiche instead.
I agree with MStodd that it will certainly give a different flavour and texture to the sauce, but if you're just looking for a healthier base to a pasta sauce, I'd say it's fine to substitute. Also as daniel mentions, you have to be careful not to overheat it (although the same is true of cream), as it will curdle.
In my experience, creme fraiche is great to make a very quick and easy pasta sauce. Put it in right at the end of cooking, once everything else is properly cooked and just heat enough so it is warmed through.
It comes down to whether you are wanting to achieve the same flavours though - if you are, maybe you could try what I would call single cream (not sure what it would be called in the US), and thicken the sauce with a bit of cornflour mixed with cold water.
Best Answer
The purpose of vodka in a cream sauce is to enhance the flavor by allowing spices that are not soluble in water to dissolve into the ethanol contained in the vodka. A suitable substitute would be any mild flavored strong alcohol.