>I can't say I've ever heard a VI chord of any sort used on a Dorian tune, but I'd be curious to know how a simple VI-minor or VI-minor7 would sound in a Dorian context.

Vaughan-Williams version of Greensleeves in the London Symphony uses both
a 1m to a 1V (major) and a bIII (major) which gives the tune a relative major-relative minor
harmonic progression. If you view bIII as a I (major) chord, then the root of the dorian mode becomes the relative minor. (VI minor).

The problem is this, the mode is never pure in most music if you set a harmonic background to it. The harmony used often suggest a modal cadence or progression,
though.

>The answer I suspect, as is often the case, is probably all to do with context.





>Therefore, when you see it (Dominant seventh) employed in any context, you know you are dealing with functional harmony and not modal harmony.

Here semantics gets in the way. Tunes are mostly hybrids suggesting modal cadences and functional conventional harmony.


>I can see that a Tune in the Lydian mode could conceivably use a dominant 7th as its V chord to help establish its tonal centre, but the problem with that would be that the Flat7 of the V chord would correspond to a natural 4th in relation to the tone centre of the tune.

>This is of fundamental importance as the only thing which makes the Lydian mode different to the major scale is the sharp 4 (#11).

There are no pure modes in jazz. The only way to define a pure modal context is to eliminate all but the basic notes of the mode in question. This certainly is not part of the jazz expression.


>I would therefore argue that adding a dominant 7th onto chord V in a lydian tune would render it functional and not modal.

Here we have to distinguish between what is functional or modal. An analogy would be any system that is restrictive or inclusive. But the inclusive system does not vitiate
the use of a restrictive device.

In short, you can have a modal cadence within a larger harmonic context.