Vigils Keep and Amaranthine

If Vigil's Keep and its men are fully upgraded then it will hold against the darkspawn attack while you save Amaranthine. So basically select the choice to save Amaranthine when the decision must be made.

I cannot remember if it counts as a requirement but make sure you remembered to close the darkspawn tunnels in the Keep's basement. If you've done all the quests, this should already have been done so if you have, you should be good. I also think you have to do the personal quests of the companions that you leave at Vigil's Keep.

Do note that Dragon Age II's import do not always register Vigil's Keep and Amaranthine as both having been saved due to an import bug. This bug registers only one of the two as having been saved - I believe it is Amaranthine. This can be fixed for Dragon Age: Inquisition through the Keep feature.

Additionally, the import for Dragon Age II may flag any companion that stayed at Vigil's Keep and survived as having died so you may not be able to acquire Nathaniel Howe's quest in Act 2 in Dragon Age 2. Instead you will recieve a quest called "Fool's Gold". If you want to see the "Finding Nathaniel" quest then I recommend bringing Nathaniel Howe with you as a party member for the final battle instead of leaving him at Vigil's Keep if that was your intent.