Hi Alex357,
The way which VTAC offers work is that any preference you put down is considered by the relevant university. So you might put down 8 preferences and you could be technically "accepted" into universities by 5 of them. Then VTAC looks at those 5 offers and sends you only ONE offer (which ever one was highest in your list of preferences).
In other words, putting Medicine first won't have any impact on whether you get offered your second/third preference (unless you get into medicine and if you are sure you won't then it's no big deal). As long as your second highest preference (after your medicine preference/preferences) is Radiography. So if you put Radiography at Monash next and have a high enough ATAR to be considered (with Monash Guarantee, etc) then you're pretty likely to get an offer (the fact that you put med first will make no difference to the way which the university considers you for Radiography).
A problem only arises if you didn't put Monash Radiography on your preferences list at all (or there is a course that you put higher up on your preferences list which you are going to get into). In that case, you will have to simply switch it to a higher position on your preferences after the first round (I'm not sure how likely this course would be to give second round offers so I can't give much information on that, sorry)
Hope that helps a little