Is there a vba equivalent to excel's mod function?
You want the mod operator.
The expression a Mod b is equivalent to the following formula:
a - (b * (a \ b))
Edited to add:
There are some special cases you may have to consider, because Excel is using floating point math (and returns a float), which the VBA function returns an integer. Because of this, using mod with floating-point numbers may require extra attention:
Excel's results may not correspond exactly with what you would predict; this is covered briefly here (see topmost answer) and at great length here.
As @André points out in the comments, negative numbers may round in the opposite direction from what you expect. The Fix() function he suggests is explained here (MSDN).