Safe to make liqueur from tomato vines?

This would be a really, really bad idea. The alkaloid compounds that make nightshades toxic can be toxic even at low levels, and a few of them are specifically alcohol soluble. This means that while chomping down on a tomato leaf might not hurt you, the toxins are readily extracted in alcohol, so you'd be maximizing your exposure to them by making a liqueur.

Further, most of the aromatic compounds in the tomato leaves and stems are going to be very bitter anyway, so even if this weren't dangerous, it would be of dubious culinary use.

Tomato Toxicity from WikiPedia: http://en.wikipedia.org/wiki/Tomato#Plant_toxicity

Source: http://www.anaturalway.com/NightShade%20Alkaloid%20Toxins.html

According to Harold McGee writing in the Curious Cook column of the New York times, despite widespread belief that tomato vines are poisonous, there is little actual supporting evidence that they are in fact poisonous:

[T]here’s scant evidence for tomato toxicity in the medical and veterinary literature. I found just one medical case, an undocumented reference to children having been made sick by tomato-leaf tea, in a 1974 book on poisonous plants. In contrast to the few anecdotal accounts of livestock poisoning, a controlled study in Israel in 1996 showed no ill effects when cattle ate tomato vines for 42 days.

That is not to say that infusing vodka with tomato vines will taste good. In my opinion, if you want concentrated tomato flavor, use concentrated tomato (paste, or sun dried, for example). It seems highly likely that this going to be far more effective than trying to create a tomato extract from the green plant parts.

You can also take a pointer from the dish "penne a la vodka" which, while trendy, shows that some flavors are more easily carried in alcohol than in water or fat, and so having some alcohol in your recipe (from wine, vodka or whatever) can help intensify the flavor.