Flames of Vengeance is an add-on for D2: Ego Draconis. This means that the story will continue after the original game. So you play FoV after D2:ED.
The dragon knight saga is D2:ED + FoV and some additional content. These extra contents are:
(Posted from another thread by Lar:)
Enemy placement and balancing (massive overhaul)
Loot and set-items (complete overhaul)
Story and scripting changes (e.g. in DKS flying fortress gameplay is very different to ED)
Art changes in Ego Draconis (though you do get the advantage of better lighting & shading)
In general, if you played or finished Ego Draconis and don't want to start over again, get FOV.
If you don't mind starting over again, get DKS, in which the total gameplay experience is in my opinion much more stream-lined.
There's no big differences between the FOV content in DKS and the FOV content applied to ED.I hope this answers your question a bit
