1- Shimano parts have had their part numbers stamped (or otherwise indicated) on them for many years. Look on the backside of the inner parallelogram link/plate. This will tell you what exact model/spec your der is.
2- Most likely if your bike has a triple crankset the der is a long cage ("GS" in Shimano nomenclature). You could count out the cogs and ring teeth to get the numbers (chain wrap and max rear cog) to confirm. Either way it is generally "safer" to err on the side of a longer cage/chain wrap spec then a too short a cage.
3- "Light Action" is, IIRC, the term used for a reduced rear der parallelogram return spring tension. Meant to be used with low friction cables and casings. Again, IIRC, this is a 7 cog "feature" that in time lost its curb appeal. But the cog count is an important detail as it will dictate what der would work with the rest of the system if the system was indexed. Pretty much any Shimano compatible 7, 8, or 9 speed rear der will index properly with the rest of the system. If the system is a friction lever one then this is of less importance.
4- As to vintage era stuff- is the system indexed? This is important as if so then the SunTour ders of that era won't work well.

The simplest way to insure a good working replacement is to get the exact same part. To veer from that one needs to understand compatibility issues. Andy