Hello.
A VOR airway is a great circle track, because of the fact that the radials are straight.
From knowing how a VOR works, the VOR ground station (transmitter) can be aligned/adjusted so that the 360 radial (for instance) is aimed in any direction, however it is common to aim it at magnetic north. Thus, the nomenclature of a radial normally coincides with their magnetic direction at the VOR site, at the time the VOR was built/last adjusted.
As you find yourself on a radial, the "local magnetic direction" of that radial will change with a change in lat/long and magnetic variation. However, the radial, itself, is per definition straight, although various propagation errors may cause it to meander, but that is another story
