Every boat I've owned has come with several random pieces that I've looked at and wondered "what in the world would you possibly use that for?"
Until I've figured out for sure what it was used for, and whether or not the boat still had whatever other pieces were necessary to make it
work, I always kept all of them.
I owned a
Columbia Defender for about 5 years before my
current boat. I was sitting in the
cockpit one day sometime about year 4 of
ownership, and my eyes rested on some random molded bit. In bit of freak inspiration, I suddenly knew exactly what another random
bronze fitting that was sitting in a bag under the settee bench was supposed to be used for - well, besides taking up valuable
storage space under the settee bench.
I wish I could now explain what it was that I finally figured out - but I've since forgot entirely. All I remember was the elated feeling of actually figuring what that weird
bronze thing was supposed to be after owning it for four and half years.
If it doesn't take up too much space, don't get rid of it until you know for sure what it really is, and that it's completely un-needed anymore.
So the roller-furler vang attachment: you now know what it is, you know your boat no longer has
roller furling, so you can get rid of it. (You also now know a little bit more about the
history of your boat: chances are that it once had
roller furling, and a PO forgot to throw that piece out when he replaced the boom.)
The other pieces though? Don't get rid of them until you know exactly what they're for.
If you get rid of them before then, you'll probably find out that they're used to keep the
keel from falling off when the
wind is blowing exactly north and it's raining.
Which, of course, you won't realize until the
wind is blowing from the north, it starts to rain, and it suddenly hits you "that's what those doohickeys do!"