If you've been around a while, this is the same old story from me. I've been happy with the cost vs. performance, not to mention ease of stripping and splicing Dyneema or Spectra core, polyester cover lines. I'm using Samson Warp Speed and Yale Maxibraid Plus on halyards, plus NE Endura Braid for spin sheets. Have not tried VPC but it looks like a reasonable and less expensive alternative. If you do strip the cover, look for one of the newer lines that have a coated core, which should last longer without getting fuzzy.
Halyards wear near the splice and at the clutch. You can extend their life by cutting 12-18" off the shackle end and re-splicing when you start to see wear. This also relocates the wear spot of the clutch. Last winter I did my first strip cover crossover, which was easier than expected, after which the core takes a simple 12-strand eye splice. If you replace your halyards with messenger lines during the off season they'll last longer, plus it gives you a chance to inspect and do repair work.
I've seen a couple J/30s with no shackle for their main halyard. They tie what is sometimes called an Australian bowline or "snot" ("it's not a knot") a half-hitch and figure-8. Using a knot instead of a shackle guarantees wear, but when that happens you only need to cut off a couple inches from the end.
Definitely examine and if need be replace your sheaves--halyards are expensive. You can go smaller but 5/16 and 3/8 hold well in our rope clutches and are easier on the hands.
