To be honest the reason i stopped doing these super high alc fermentations was the slow seemingly endless fermentation at the end,I now just add cheap brandy after about 15% natural fermentation.
I have kept some ferments going for months.
At 3 to 4 bloops per min its not finished yet !!!
I dont personally know the yeast you used but it must be a "good un".
If you are interested in "fortification" I find brandy much superior than vodka for this purpose.
It may be a grape/grain thing.???
You could once the wine has cleared and been racked fill up the airspace in the demijons with brandy.
It will after all be a "sipping" wine
My main interest in high alc fermentations at the start (about 30yrs ago) was curiosity about how far i could "push it"
plus i do like the taste of port/sherry.
If i ever get round to building an estufa i might have another go in the future at a Madera clone.
A bottle of this would be a nice thought,Thanks !!
Keep in touch as its nearly time for your next one.!!!!!!!!!