Q. How strong does the vodka have to be in order to make it shelf stable? I heard it was at least 40 proof but not very sure.
A. All vodka sold in the US must be at least 40% alcohol which translates to 80 proof.

Q. Do ripe plums work better or should they be fairly firm?
A. I tend to use fruit at the peak of flavor/ripeness, but overripe fruit is not necessarily bad. The riper fruit tends to be a bit harder to strain out of the alcohol to get a clearer end product, so you'll lose a bit of volume the more you have to strain it.

Q. Do I cut the plums to segments or plunge it whole?
A. The bigger the pieces, the easier to strain at the end. I usually cut them at least in half depending on the size of the fruit. But it's up to you. No hard and fast rules.

Q. What about the seeds?
A. Yes, any stone fruit should be pitted.

One "recipe" can be found in the Jamlady Cookbook:

"As a rule of thumb, a pound of any fruit or berries can be put with 3 cups of vodka (or equal parts of grain alcohol - also called Everclear - and water) plus 1 to 1 1/4 cups of sugar to make a fruit liqueur."

I like to add a 1/2 to 3/4 cup of brandy as well. Any spirits can be used - gin goes well with plums if you like gin :-) Mostly I use vodka due to its somewhat neutral taste.

Hope that helps.

Don't forget to eat the plums! LOL

I know a guy that makes some "kind of illegal" whiskey with all kinds of fruit. It's only illegal because he makes too much according to the Feds. He doesn't sell it, he just likes to share!

I agree with Malna, remove the pits, they can make it bitter.