decimal to fraction

Don't you need to format as fraction the cell where is = koshent(numerator, denominator)
BTW
you could use

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I'm sure there is a way to do it with VBA. Perhaps if you explained the overall context and why you want to do it in VBA.

One thing will be sure, a UDF cannot change the format of the cell it resides in. If the UDF result will not be needed in other calculations, you could dimension the UDF to return a string, then use text manipulation functions to return a text string:

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The problem that will usually come from this approach is that the result of koshent is a text string, not a real number, and any calculations that depend on the result of koshent will need to convert the text string into a number.

If you would like, you can use a separate VBA Sub procedure to format the cell as fraction ("0 0/0") using the .numberformat property of the cell. I'm not sure why this would be an advantage, though.

I think we will need a broader context to understand why you want to do this in VBA. IMO, the simplest approach, like with built in functions, is to have the UDF return a double like you have it, then use number formatting like amorshc and PCI suggested.