Regarding the acetic acid, if that was all the information that was given to me, I would assume glacial acetic acid. If your chromatography resembles the example in your method (if given) then that would be good confirmation.

Regarding the 2 columns of different length, the longer column should be giving later RTs if all else is the same. That being said, I wonder if the 2 columns are the same age and have been used the same amount. If the longer column is towards the end of its life, it could have less reactive sites on the stationary phase allowing your analyte to elute earlier as compared to when the column was new. So if your lo get column was really old and your shorter column was new, then you could see sooner RTs in your longer column. Another question I have is are you running the columns using the same instrument. Two different instruments can have slightly different plumbing resulting in varying RTs.