Texture Alpha...

How can I go about manipulating the alpha of a texture at each vertex of a poly at run time?

Should I use the glTexEnv functions? If so which parameters will make this work?

Thanks in advance!

You need to use the alpha associated with the vertex color, then select the appropriate texture environment to adjust the transparency. Depending on your intent you may need to use combiners or the equivalent functionality.

P.S. one texenv that would do this is GL_MODULATE.

Look at the table in the manual. Anything that puts A=Af or A=AfAt will do this and most of the options do this, your choice depends on your texture format, what you want the rgb to be doing when you’re blending alpha and whether you want to add texel alpha in addition to vertex alpha.