MSOf10 Hiding Formula in a Field defined at Text

I have two possible user entered fields - one is the company's part # and the other is the manufacturer's (mfg.) part #.

If the form's user enters the company's part #, the mfg.'s part # field is populated using a VLOOKUP formula.
If the form's user enters the mfg.'s part #, I want to populate the company's part # field upon submission. (prevent circular reference error)
My problem is if I define the mfg.'s part # field as text, the search for the matching mfg. #s work, but I cannot hide the formulas. If the mfg.'s part # is defined as General, the formulas stay hidden but the index/match will not find the matching mfg.'s number when the mfg. part # is all numeric. Only when the mfg. # is text (numbers and letters) the match works.

I do not want the formulas showing, as it will be confusing to them and they will not intuitively know they can enter data in the mfg. part # field.

Is there a work around?