Watch the Southern Rock Vamp online guitar lesson by Jeff McErlain from Blues Guitar Survival Guide - Lead
The parent scale of this chord progression is mixolydian. The formula for a mixolydian scale is R, 2, 3, 4, 5 , 6, b7. Mixolydian is a major scale with a lowered 7th, a lowered 7th just means we change the distance between the 7th note of the scale and the root by a half step. In a regular major scale (Ionian) there is a half step between the 7th and the root. In mixolydian there is a b7 between the 7th and root. Is we look at our scale formulae in the previous section we can see that when we mix a major blues scale and a dominant pentatonic scale we get R, 2, b3, 3, 4, 5, 6 b7. Basically a mixolydian scale with an added b3rd, which is the blue note in a major blues scale. If we omit that note it gives us a straight mixolydian scale. See you have been using the modes and didn't even know it! My main suggestion is to take in the sound of this vamp, it's mixolydian.