From my own experience, nothing BAD will happen. You may feel a little more elated or feel the drink more when mixing the 2. However this also depends on how long you have been taking valuim for - the longer you take it the less of an effect it will have when mixing the 2.
On a different note - my experience with being prescribed valuim has not been a good one, its considered a "pseudo-narcotic" and even thu I need it for anxiety all doctors just want to see me off of it. Personally I feel they just don't wanna take the personal responsibility for having prescribed it. What has your experience been??