There are a few sources of vent liners... I believe I sell the best ones: 17-4 stainless, heat treated, and made in the USA by Lehigh (aka Millennium Manufacturing). How many shots depends on a lot of factors... first what do you deem too big of a flash hole (typically they start around .030 and when they grow to .036 you are either just starting to see your groups open up, or about to). But there are other opinions on flash hole sizes (Where to start and when to replace). And how big of a group change is acceptable.
You should get 100-150 shots on one of our stainless vent liners... the other options aren't always heat treated or aren't made out of steel that will take as many shots. I have one customer that claims to have over 300 shots on one of the newer stainless vent liners and still can't get our .036 pin gage through it. But it really depends on so much one cannot tell you how many shots... how are you cleaning them? What brand of primer are you using? what powder? How much powder? How high of pressure are you subjecting them too? Are you letting them cool between shots? etc.
To me, the fact that many manufacturers would rather sell you a new breech plug than to use vent liners (or another replicable flash hole system) is a big tell.
Any yes, the Lehigh vent liners work great in converted breech plugs (assuming the work is done correctly and their is enough steel in the breech plug for it to be converted). They are essentially interchangeable with the standard Savage ventliner and are the most common readily available type.
Hope to help.
Tom