If my understanding is correct:
1. You're system is already waterproof, because you are checking those cells before print, save ... If not the user gets a warning.
2. What you want is some additional feature to put the attention of the user to both cells which need to be filled in.
If the user may not get an inputbox, how do you see this happening?
What do you think of the next suggestions?
1. when sheet is activated and the date is not filled in, it will be selected
2. till it is filled in, it is red
3. till it is filled in, al other cells are locked
4. using another cell with formula like = IF(A1="","Filll in data","")
5. this cell can also have conditional format
same story for vendor
instead of another cell, you could use a label which would get invisible when data and vendor are filled in
So: what is your scenario?