I use a widescreen monitor, so will VB-M work for full screen on that? Doesn't sound like it will, but. The one time I tried it, it changed my resolution and I couldn't see anything to fix it... didn't even have a chance to load anything up.
Options > Gameboy > Border
This will create a border which I believe gives you the correct aspect ratio (but not in fullscreen).
Then just reduce your system resolution to increase the size of Visualboy Advanced to be as close to full screen as possible. Consider using 2x VBA size or 3x VBA size to find the perfect size.