Are second/vacation residences subject to Reg. Z?

Reg. Z is purpose driven......so if the loan is secured by a 2nd or vacation home and the purpose of the funds is consumer purpose, then yes, they are subject to Reg. Z.

A loan to purchase a 2nd or vacation home is most definitely subject to Reg. Z. A loan to a rancher to purchase cattle secured by his vacation home would not be subject to Reg. Z.

You have to look to the purpose of the funds.

You also have to look at the type of borrower (which was not mentioned in the OP.) Regulation Z applies to loans to consumers. Corporations and other non-consumer borrowers are not entitled to Reg. Z's disclosures and protections.