3 days before my mom got married my stepdad finished chemo. About 5 minutes in he started sweating and looking a big weak so they gave him a chair. My mom now towering over him got on her knees. It was such a beautiful moment as none of us expected it. If he cant stand (but wants to try to as long as he can) just make sure theres a chair close by someone can grab for him and just get down on his level. It sure looked romantic when my mom did it
Well for my ceremony we took all but 15 minutes to say our bits and exchange rings. We just signed the official papers right after the ceremony with just our wedding party present (including our witnesses).
Perhaps you could just do your own personal vows at a first look or write them down and exchange letters for the morning of?