You need a framework to market make this I think, on the sell side. On the buy side, it's whatever you want. What we all know is that at the forward expiration date, it will be an 'observable' ATM spot vol contract.
I saw this being bid-offered off of a vanilla swaption and a mid-curve. So as overkill112358 was saying. You would break this down into vanilla swaptions and a correlation exposure. I would not just sit on it. To the extent possible you can hedge the spot vol and you can hedge the correlation using yield curve (CMS) spread options. It's a little hairy to hedge the correlation.
In any case, you are better off making markets in such products if you have a consistent a framework as possible. I think it is possible to do that, but sure this will be subject to underlying statistical assumptions, which you may or may not like.