This is an older question, so I assume the chips have fallen where they may; however, I am sorry I missed the question because this is important informaiton to share.
NO! A seller does not have the right to direct who a buyer works with for financing. Goodness knows, builders insist this all the time; however, there is no law or requirement that a borrower follows their directive. That said, the seller does have every right to refuse an offer of purchase. The seller/listing agent may have inside knowledge on the lender a borrower is working with and have reservations about the lenders ability to close the deal. In that case, they would probably say "get a new lender".
There is very little of a mortgage file that cannot be "reproduced" quite easily, so there is virtually no need for your current lender to release the file to a new lender. The exception would be VA, FHA or specific programs that have a registration number....since you are buying a vacation home and none of these programs are applicable, this would be a non-issue.
All the new lender needs to do is get updated financials from you (after five months they are needed anyway), get the title and purchase contract from the title/escrow company, order new credit (again, needed anyway)...you will probably need to buy a new appraisal....again, needed anyway. NOTE: All documentation is a mortgage file must be no older than 90 - 120 days...and most lenders in today's market want everything within 30 days. Therefore, virtually nothing the original lender has is of any value for the purpose of closing a loan.
Hope this information helps anyone who is in the position of needing to/or considering switching to a new lender.