You are exactly right.
On a side note, if it is several trips you are trying to "expand" into vacation and if you exceed 54 CH (8 days x 6 CH = 48 + 6 add'l vacation hours permitted), then the new vacation menu will decline the expansion (first hand experience)....EVEN IF YOU HAVE MORE THAN 54 CH LEFT IN YOUR VACATION ACCOUNT.
Looks like buy-back for me this year.
However, if it is TWO pairings, then you are ok to expand over the 54 CHs and knock both pairings out for the month.