Since you can't find another vendor, it might be time to bring on software consulting practice that specializes in this area or at least doing something similar.
If you can find a firm that is capable of doing the work, then you can figure out how you will maintain the software. You'd have to determine whether it would be better to engage that firm for the maintenance or hire someone. Possibly a combination of both. This could be done while the software is being built.
Large software projects are more likely to take longer than estimated and go over budget, so be sure to factor this in to your calculations.
If you can find a firm that is capable of doing the work, then you can figure out how you will maintain the software. You'd have to determine whether it would be better to engage that firm for the maintenance or hire someone. Possibly a combination of both. This could be done while the software is being built.
Large software projects are more likely to take longer than estimated and go over budget, so be sure to factor this in to your calculations.