My guess is that you should make the HexDraw map, export it to some common image format, then make a VASL map out of that.
If you just want to use a HexDraw map in VASL without taking advantage of the VASL color-based transformations (like changing Open Ground to white in Winter):
1) First you'd need to open the HexDraw map in some image editor, scale it to VASL size, then export that to a png format (png enables you to keep all the colors in your scaled image, ensuring it looks as sharp as you see in your image editor. Gif format would reduce the number of colors to 256, which can look remarkably bad under certain circumstances). The way to ensure it's scaled to VASL size is to measure the x and y distances between certain points (say, the top of hex A1 to the bottom of hex A10, and the leftmost point of hex B1 to the rightmost point of hex FF1). Do this for both the HexDraw map and for a standard VASL map. Using your image editor, scale the image so that the HexDraw distances match those of the VASL map - the scaling factors might be slightly different in x and y; don't worry about that.
2) Once you've got your rescaled HexDraw map with a png extension, change that png extension to gif. Yes, your OS will complain, but do it anyway. You won't be changing anything about the file itself, other than the extension. VASL knows how to handle bdYourBoard files (which are actually .zip archives without the .zip extension) that have a "bdYourBoard.gif" file inside them, but I don't think it understands what to do if it only sees a "bdYourBoard.png" file.
3) Create a text file called data (without any extension). The first (and only) line of the file should just say "v1". That's all you need.
4) Make a .zip archive file out of the two files above (data and YourBoard.gif). Don't use any compression; just choose the "archive" or "save" option. Once you get done making the archive, rename it to bdYourBoard, without any extension.
5) Put bdYourBoard in your VASL boards folder and open up a new VASL and create a New Game. Your new board should be available in the board Picker dialog.
Note that auto-LOS will not be enabled, but the LOS string will always be there. To add auto-LOS to a VASL board, you need to talk to Doug Rimmer.
If you DO want to convert a HexDraw map to VASL with fully-enabled VASL colors, there's no getting around the fact that you're gonna have to redo the artwork using standard VASL colors and patterns, as described in the Ye Olde Booke. You'd still have to rescale the board to VASL dimensions as described in #1 above, but at that point you'd have to simply use the HexDraw board as a template for redoing the artwork with VASL colors. Fortunately, once you're using VASL colors, you should be able to save as a gif, because you're most certainly not going to be using up the full 256 color palette that gifs are capable-of/restricted-to.
If you want to do this a lot, then it's worth getting good at the process. Course, IMO if you're going to invest the time and ultimately want a VASL-enabled board, I think it's more efficient to work with VASL colors and artwork directly, rather than go through Hexdraw, but that's not to dis HexDraw, which is amazing. If you only want to do this in a one-off fashion, say as part of a HASL you're developing, it's probably worth handing the work off to a VASL elf; they don't charge much, and can be an absolute joy to work with.