try changing like
tot = instr(1, thebody,"TOTAL AMT")
if tot > 0 then 
  mynumber = mid(thebody, tot + 9, instr(tot, thebody, "VENDOR") - tot - 9)
  else
  msgbox "TOTAL AMT not found"
end if
if it still fails make sure that instr for Vendor is not = 0, though it may not matter if it is
this will also be quicker (though maybe not noticeable), than 3 calls to instr total amt