Sorry your terms are messing me up I think... let me break it down to what your saying to me so I understand better..
SSD and flash drives arnt the same, they are totally differnt type of tech. While they use the same storage (flash) they are differnt on many levels, chips, size, speed, exc...
1 Installed Xp on the C drive
2 Installed Vista on the G drive
==ok
3 You installed eboostr on Vista 1st.
=ok
4 I made the usb my cach device using eboostr
=ok
5 Next I installed eboostr on Xp
=ok
6 Caused I used the usb on vista. I changed it to system memory?
=not ok, I dont understand this step you took. If your usb was full when you said you used it for your vista then you cant change it to something else otherwise you are in a way took the information on it and deleted it.
7 The usb changed into a blank drive again when I went back to vista
=Yea, cause you deleted the information that was put on the flash drive by vista's eboostr
The reason you cant use the vista's ebootr information on the xp is cause the 2 OS are differnt in code, size, placements, exc... So you cant really use the same usb or flash drive unless you share/ partiton the drive, eboostr does this auto magicly for you so unless you use up all the flash drive you can do it. Aka 2gb flash drive, 1 GB for vista and the other g for xp will work. So using up all 2 gigs for vista then deleting it for xp to use up the same 2gbs is pointless and actully would increase start up time be alot. Besides killing your flash drive too... Unless your meaning something else I really dont see a bug or a installing problem, but again I could be wrong on what your actully trying to say..