If you're using 0.10.1, you can do :file on an Excel file to see if vifm is going to run the command or skip it. Next to excel %f & there will be no [present] mark, which means that vifm didn't find excel in %PATH%.
To fix this, specify full path to the excel executable. Something like this (don't remember if .exe suffix is actually required, might be optional):
filetype *.xls,*.xlsx "c:\program file\ms office\excel.exe" %"f &
You probably need %"f instead of %f or spaces in file names might not be handled properly.