Head to Batallia Downs (S) as your level 30+ job and go to (J-10) to enter The Eldieme Necropolis (S). (If you have been to The Eldieme Necropolis (S) before and have allied yourself with one of the three nations in the past, you may instead speak with your nation's appropriate Campaign Arbiter to be teleported directly to the zone.)
After talking to Erlene, you can talk to Tucker (K-8) in Crawlers' Nest (S) and flag a mini-quest to obtain one sheet of Vellum for every 12 Rolanberries traded to him. Note: you MUST talk to Erlene or Tucker will not trade.
Note that Tucker will only take up to four stacks of Rolanberries per trade and will only trade with you a total of three times. This means that to get the stack of Vellum you will need to trade him four stacks of Rolanberries three times.
If your current job is not BLM, RDM, SMN or BLU, change jobs to one of those. The job level does not matter, and can be lower than 30.