I havent touched the game in like a year so my memory is hazy as hell but if im recalling correctly
if you somehow got 5 members without it crashing? then no clue
but if you mean to avoid it from happening while switching characters? you should be able to trade an empty space for an in party character. it was either that or you just have to remove the in party character first then add the new one as two separate actions
in the case that i am wrong
i apologize for both being wrong
and not wanting to re-download and relearn the method myself
so you talk to the guy by the entrance of the brig
party on the left storage box on the right
when you select a party member to switch with the storage box
there is an invisible empty slot it will always be at the bottom on your storage box character list. that is how you deposit a member directly into storage. Once the party slot is freed up you select that empty slot on the party and select the member from storage. its a round about why to do this but it avoids that weird crash from happening. as for the Vine thing i cant remember.