I have done this MANY times (bid Reserve during vacation months). Here's my reasonably certain opinion ...

After you "extend" your vacation, your R line will be totally rebuilt around vacation days, perhaps (probably?) having none of the original R days on the R line.

So ... you extend your 8 days to 13 days (in a 4 week bid month you'll owe them 14 days, so you owe them only 1 R days). I've had good luck asking "Futures" for the R days I desired.

Good luck ...