should I refinish laquer, leave it as is, or remove finish on a vintage trombone?

What an open ended question. There is no correct answer.

If your goal is to make the instrument last as long as possible and look nice, yes, have it refinished. Most shops today use a baked epoxy finish that is thicker and longer lasting than the original cellulous lacquer. However, the dent and scratch removal in combination with the buffing process can sometimes take away a lot of the original metal and engraving.

If your purpose is to have the instrument sound as authentic as possible leave it as is. That may also preserve its value to collectors the most.

Although I do not agree with it, I have stripped lacquer for a lot of players in my time. Some of them do prefer the sound that way. You will have to learn to keep the horn clean and wiped down or it will look like puppy du-du. On old horns, the lacquer is usually pretty easy to strip. Sometimes hot water will do it. Because of the dangers involved, I recommend a repair facility to remove it if chemicals are needed.

Also, as has been noted in other forums at this site, many people are allergic to brass so be careful.