That's why I said "that's effectively what's being voted on". If the winner is a MSP, that's the recommendation of the SNP members that that person should be DFM. Every UK Parliament has seen the party's deputy leader as the deputy leader of the government - expect in the situation of a collation.
The DFM role was only created for the collation between Labour and Lib Dems between 1999 and 2007. In 2007, Salmond appointed his depute as the DFM.
In the event of the FM being unavailable for a prolonged period of time - the DFM becomes leader of the Parliament. If the SNP haven't appointed a DFM, then it goes to the depute of the party.
Apart from not being required (IMO) I am fairly sure that DFM isn't even a formal position within the Scottish government. I think the same applies to the position of Deputy Prime Minister at Westminster. There have been significant periods of time when we didn't have a Deputy Prime Minister.
But the point is that if Angela Constance were to get elected (which she wont) there is no reason why she can't be Deputy SNP leader without the requirement to be DFM. Nicola Sturgeon could choose not to have a DFM or appoint anybody that she chooses.
SNP members are not voting to appoint a DFM. That will be Nicola Sturgeons job after she takes over as First Minister.