The speaker doesn't say that he actually saw his deceased wife. He says he thought he saw her, which opens up quite the can of worms. Was his vision an imaginative one, created by his own mind? Or did someone actually appear to the speaker? If so, was it his deceased wife, or something else entirely? He's hedging his bets here, and with good reason. When's the last time you thought someone who saw a ghost was sane?
Brought to me like Alcestis from the grave
Mine, as whom wash't from spot of childbed taint,
And such, as yet once more I hope to have
Full sight of her in Heaven without restraint. (2, 5, 7-8)
By using a series of similes to describe the speaker's vision, rather than describing it directly, the poem suggests that what the speaker saw was like his deceased wife, but it wasn't exactly her. Since this vision was only "such, as," the speaker anticipates his wife will be in heaven, but he really can't say for sure.
But O! as to embrace me she inclin'd,
I wak'd, she fled, and day brought back my night. (13-14)
With these lines, the speaker wakes up to find himself in a completely different reality than the one in his dream. In his dream, he was able to see the spiritual qualities of his wife's person, as if she were really there. In this world, the speaker is blind and his wife is oh so gone. "Day brought back my night" both emphasizes the speaker's blindness and comments upon how that blindness, and his state of mourning, make his reality different from everybody else's. For him, day is night. And not in a good way.
I wak'd […] (14)
Here is our first indication that what the speaker experienced was a dream rather than an imaginative vision or spiritual visitation. This idea throws yet one more alternative reality into the mix.