Your A Dorian vamp needs to:Originally Posted by Lowthorpe
1.) establish the root (A)
2.) establish the minor third (C)
3.) establish the 6th (F#) as this is the characteristic note of A Dorian that distinguishes it from the other minor modes
If your chords are Am7 to D7 then that should work fine.That's a common A Dorian vamp.The D7 wants to go somewhere,namely to the G chord.But if you go back to Am7 instead then it sounds like you meant to do it.It sounds as though you've started on Am7 and resolved back to Am7.Therefore it sounds like a looping vamp in A Dorian.It also meets the three requirements for establishing a vamp above.To make a vamp for any mode you must establish the root,the third(to tell that it's major or minor),and establish the characteristic note of the mode.
To determine what the characteristic note is for a mode simply compare major modes to the major scale or minor modes to the natural minor scale.They will only differ by one note.
major scale = 1 2 3 4 5 6 7
lydian = 1 2 3 #4 5 6 7 (different note = the #4)
mixolydian = 1 2 3 4 5 6 b7 (different note = the b7)
natural minor scale = 1 2 b3 4 5 b6 b7
dorian = 1 2 b3 4 5 6 b7 (different note = the 6th)
Phrygian = 1 b2 b3 4 5 b6 b7 (different note = the b2)
locrian has two notes that are different (the b2 and b5)
locrian = 1 b2 b3 4 b5 b6 b7
locrian isn't used a whole lot so don't worry too much about it