Hard to say without more info. There are a number of variables that can affect the relative height of the handlebars. A big factor that a lot of folks forget to take into account is bottom bracket drop—a lower bottom bracket effectively lengthens the head tube, and vice versa. So if the bottom bracket is lower on the Vamoots than on the Roubaix, then the handlebars may be closer to the floor but, all other things being equal (which of course they generally are not), the saddle-to-bar drop will be the same. Another factor is stem length—the longer the stem, the more the rise (assuming that you have your 8º with positive rise). Other factors include the axle-to-crown height of the fork, the stack height of the headset, and the number of spacers that you are using.
Personally, if I were starting from scratch, I would try to solve for the headtube height I wanted assuming a -6º stem (just because I prefer the way it looks vs. a positively sloped stem) and approximately 2cm of spacers. To get there, I would think that you would need a slightly taller headtube, in the 16 - 17cm range that you mention. I measured two bikes, both with 8cm of bb drop and 110mm stems, but one has a 16.3cm headtube and the other a 17cm headtube (the fork and headset heights are roughly the same). They both have handlebars close to 38" from the ground. The one with the taller headtube has 2cm of spacers. If you stick with the 15cm headtube, you could probably get there with an 8º rise stem and/or more spacers, but I think that a modest headtube extension would be the better way to go.
If you want to get a more precise rendering, plug the numbers in to BikeCAD, or ask your Moots dealer to do a mock-up in BikeCAD for you (they should be able to do this easily). My recollection is that Moots didn't charge extra to extend the headtube, but Jon should be able to confirm.