3/5 of vacation time - yes, why not? That's standard for part-time people here in the UK.
We obviously don't have the same healthcare situation as you, but surely it's just part of the negotiation? If 5 days a week is worth a $100k salary and $10k healthcare (don't ask me if that healthcare cost is 10x too high or 10x too low... but that doesn't matter) then 3 days a week is work $60k salary plus $6k healthcare. So offer that person $54k and healthcare. Or $66k and no healthcare. Simple enough, surely?