I never heard of “understudy days,” but LBB already announced her vacation days, so she should be there when you go see her (so don’t panic! lol

). Unless she gets hurt/sick/emergency or something sudden like that. broadwayunderstudies.com may be of some help as the date gets closer.
Stage door tips? Hm.. well first, when you come out the FRONT entrance, just go right, around the corner and there it is on the right and wait behind the barricades lined up. I believe there also is a side exit, but I’ve never used that so I don’t know exactly where that is. and as for tips.. ummm… just be polite and thank each performer. Don’t be a crazy screaming obsessed fan, lol. Have your marker and camera ready. I don’t really know what to say. I just try to be calm, polite and keep it short and sweet.
Just have an amazing time!
hope that sort of helps you a little bit. and the link maya5892 provided may be of some use.