Petalshop - Extra /vault slots - unclear how this interacts with ranks

Currently, at least to me, it's not clear how purchasing extra /vault slots works when you already have a rank. If I have Warlord rank (12 rows/2 pages of /vault), would buying 6 extra rows give me a full extra page of /vault? Or am I already at my maximum number of slots? (i.e. buying vault slots in the petalshop is an alternative for those who want a larger vault, but don't want to buy a higher rank)? Another example - I have Sage (2 vault rows). Would buying 2 vault rows in the petalshop give me 2 more rows, or would it not benefit me because I already have 2 rows? It should also prevent you from making the purchase if it doesn't benefit you with your current rank (might already be the case, but would rather not test and learn the hard way) If it's not a valid purchase (i.e. doesn't stack with current rank), then I think it would be nice if it showed as "already purchased" in the petalshop or similar. If it IS a valid purchase, this could be made more clear, maybe by having it say "this will add 2 rows to your current number of rows" or similar.