okay - we have had two VCUG's. one when my daughter was 2 and one at 3. she has kidney reflux and has been on the antibiotic now for almost two years.
i wish i could sit here and say it was okay, but it was NOT. at least for us. now, we do have a friend whose little girl did just fine. so every child is different. but my daughter happens to be melodramatic in nearly every situation, so an unpleasant one is only exacerbated by her tendency for the dramatic.
your daughter will lay down on what is essentially an x-ray table. they'll put a little numbing agent on all her little "parts" and then catheterize her with a small, tiny, teeny catheter. they'll pump dye into her bladder and fill it to capacity, leaving your daughter with the urge to urinate. the idea then is for her to release her bladder while lying on the table, so that the x-ray can see if the dye is being expelled from the body, or traveling backward up the urinary tract toward the kidney.
my daughter has refused, amid screams from her and pleads from mommy, daddy, doctor, nurse, to urinate on the table. so both times we have ended up with daddy picking her up, running to the adjacent potty, letting her "go", and then RUNNING back to the table (just two or three daddy-steps away) to lay her down and get an image of where the dye crept.
my friend whose daughter did this successfully had her child lay down on the kitchen table on a towel and practice urinating while laying on a table. it eased her anxiety and served as a practice run - so if your daughter would respond well to that, go for it. mine would have freaked out even more, so it was not a good plan for us.
my best advice after doing it twice is to go to the store with your daughter, buy a toy she has been wanting, don't open it, but take it to the appointment. use it as incentive to get through the procedure, and obviously give it to her whether she does "well" or not.
i'll say a prayer for you! it's not pleasant, but it's quick.