Here is a post from May 2011: ecredits to pay for hotel via deltavacations? which info stated that Delta Vacations can only use Delta vouchers towards the air portion.

On Delta Vacations Terms and Conditions page it states:
https://www.delta.com/us/en/delta-va...and-conditions

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    • Transportation Credit Vouchers (TCV) issued by Delta Air Lines®, Electronic Credit Vouchers (ECV/E-TCV), Miscellaneous Charge Orders (MCO) or Electronic Denied Compensation Credits are accepted as forms of payment toward the flight portion of a Delta Vacations package, as long as travel begins on or before one year from the issue date and in accordance with terms and conditions on the certificate/voucher. You will have the following payment options:
    • If Hold for 24 Hours is allowed, select that option, and call 1-800-800-1504 to confirm your reservation and apply payment.
    • If Hold for 24 Hours is not allowed, select Purchase Now, and call 1-800-800-1504 immediately to apply your travel certificate and receive credit.