I have a value list based on a key number that displays a second field which is a part number, a combination of texts and numbers. I have created a new field for sorting the part number by its internal numerical value, but how can I display the complet part number and have the VL sorted by its numerical value?

You'll have to create a calculated field that concatenates the two fields that you want to display, beginning with your sort field (padded with enough zeros to make the text field sort properly as a number.)

OK, but what do I do with that field, I cannot display it in the value list because it will be confusing for the user. I have already created a calculated field that extracts the number from the part #, I use that for sorting the records on the correspondent table, but I am stuck with the value list.

use a portal instead, then you can show whatever you want, with a lot fewer restrictions, and sort by any field.