Is it obligatory to make up the fast of one of the six days of Shawwal if a questionable discharge (resembling reddish water) occurs after the Maghrib call to prayer, knowing that there is a vow to fast these days annually, and is it permissible to give charity instead of making up the fast due to diabetes?

A vow to fast six days of Shawwal must be fulfilled unless one is permanently unable to do so, in which case an expiation for a broken oath becomes obligatory. The presence of a spot of blood after the Maghrib adhan does not affect the fast of that day, and your fast is valid; you do not need to make up that day. As for yellow discharge, it has its own specific ruling.