Let's say you have dual spec and have one spec with Glyph of Seal of Vengeance and a second one without it.
What you now can do is, activate the spec with the above mentioned glyph and use either Seal of Corruption(Horde) or Seal of Vengeance(Alliance). It will buff you with the respective Seal and improve your expertise by 10 expertise points -- as it should.
Next you switch back to the second spec which does not have Glyph of Seal of Vengeance. Since dual spec does not remove Seals, as it does with other spells like Righteous Fury, you will still have your exp improved by 10 expertise points until you refresh the seal. That should not be the case.
One possible fix would be to remove Seals as well when switching spec with dual spec. This might or might not be how it worked on retail -- don't know that.
Let's say you have dual spec and have one spec with Glyph of Seal of Vengeance and a second one without it. What you now can do is, activate the spec with the above mentioned glyph and use either Seal of Corruption(Horde) or Seal of Vengeance(Alliance). It will buff you with the respective Seal and improve your expertise by 10 expertise points -- as it should. Next you switch back to the second spec which does not have Glyph of Seal of Vengeance. Since dual spec does not remove Seals, as it does with other spells like Righteous Fury, you will still have your exp improved by 10 expertise points until you refresh the seal. That should not be the case. One possible fix would be to remove Seals as well when switching spec with dual spec. This might or might not be how it worked on retail -- don't know that.