It's fairly self explanatory if you look at the parameter list. vtxs is a void* to either ALLEGRO_VERTEX's or custom vertices you create using an ALLEGRO_VERTEX_DECL object. See http://www.allegro.cc/manual/5/allegro_prim_type for what to use for the type parameter.

Click ALLEGRO_VERTEX in that code listing and it'll show you that there is another parameter, color. So That code probably shouldn't have compiled in MSVC. I usually just set them like this:

v = 0;
verts[v].x = some_x; // same for y, z = 0
verts[v].u = some_u; // same for v
verts[v].color = some_ALLEGRO_COLOR;
v++;
// next vertex

Color is important for texturing too. If you want the texture to show up without tinting, use pure white.

texels will be multiplied by the tint (in al_draw_tinted_*_bitmap) and color (in al_draw_prim).

If you have a RGB value of (0.5, 1.0, 0.25) in the texture...

Multiply by white: (0.5*1, 1.0*1, 0.25*1) it doesn't change.

If you wanted to make it half the brightness, multiply by a shade of gray (0.5, 0.5, 0.5):

(0.5*0.5, 1.0*0.5, 0.25*0.5) and you get (0.25, 0.5, 0.125) which is half as bright as the original texture color.

You could also filter out all color components but red, as another example:

(0.5*1, 1.0*0, 0.25*0) = (0.5, 0, 0) only the red color remains. And so on for any color you choose, in any proportion.

EDIT: Your example just shows the way alpha blending is done. Allegro uses premultipled alpha by default, which has the blender set like so:

al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA);

All it means is the color components should be pre-multiplied by the alpha value before drawing, which you can see is what's done in your example.

What may seem more familiar (but has problems, see the manual) is this blend mode:

al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA);

With that mode you'd use the comment out line in your example instead.

I still cannot get it to work, i dont know why.
Can you make a full example which works if executed with the prim?