Hi,
Are you trying to do this in a template then the user field naming convention would be different.
Ex: project field: user _xx
Template field: template_user_xx.
So please follow the standard way of using the naming conventions, while you make some workflow changes (Template change/Project change).
may be the below should be helpfull.
sRestCmntDtls = Bug_Fields.Field("BG_Template_USER_25").Value
if it not the case, I mentioned above. please let me know where exactly you are using this piece of code in Defect module?
Damodar Reddy