Does the boiling boint of miscible liquids follow a linear relationship? I want to create a steam bath for a reaction the proceeds well at 105&#8451;.  I cannot use pure (demineralized, anyway) water for this, since it would boil at 100&#8451; at STP.

As such I figured that maybe I could get the required temperature by mixing water with another liquid (with which water is miscible) which has a higher boiling point (e.g. vinegar at BP around 118.5&#8451; or vegetable glycerin with BP at 190&#8451;).

Does the boiling point of the mixture of liquids proceed in a linear manner between the two temperatures?  E.G. would a volume of 50% water &amp; 50% VG boil at 145&#8451;?  

As it is being done right on the boiling point of the combined liquids, I expect to lose some of the liquid to steam and need to add more. Will the lower temperature liquid boil out first?
Except if molecular composition of your two liquids are similar, the boiling point of the mixture will not proceeds linearly but like this for instance : 



With the lower blue curve the boiling curve. In between the two curve you'll then have a mixture of vapor and liquid and only vapor on the upper part the binary diagram. 

So you should find out an already drawn binary diagram for your mixture or draw it out by yourself first.