One thought...

Shoot the subject against a green screen.

Chromakey that scene over the building background. (Background on track 1, subject on track 2)

Select (highlight) track 2, then MODIFY.
Vary the OPACITY (transparency) of the subject video to begin with 100% and end with zero.

Another option: Take a lengthy shot of just the building, then take the shot of the person walking toward it. (The camera must not be moved at all during the entire process. Seriously, not one millimeter.)

Place the first shot on track one, and the walking shot on track two. Click on the track two clip, then Modify.

Under the properties tab, make sure Opacity is at 100%.

Next, (still in the Modify window/PiP Designer) under the image, you see a number of adjustments that can be made. (Opacity, Scale, Rotation, etc.) You want to create a keyframe on the Opacity line at the very beginning. This makes your image start out fully visible, as you want. You create the keyframe by clicking on the white diamond. You'll see a red diamond appear at the beginning. That's your keyframe.

Now (still in PiP Designer) move the slider all the way to the end. If there isn't already a keyframe at the end, make one.

Go back to the place under the Properties tab where you set the opacity percentage and make it zero.

The next step is determining where you want the vanishing to begin. (You're still in PiP Designer for this.) Move the slider to the appropriate place. Click the Opacity diamond to create a keyframe there. Right click on the newly created red diamond, and choose Duplicate Previous Keyframe. This is now the very last spot that your person will be at 100% visibility. He will begin vanishing at that point.

If you want him completely gone before the end of the clip, drag the slider to the spot you'd like him invisible, then create a keyframe there. Right click on the new red diamond and choose Duplicate Next Keyframe. This makes him as invisible as he will be at the very end.

You can preview it by clicking the play button, still in the PiP Designer. If you like it, click Save.

The above might look complicated, but if you actually do it you'll see it's very easy. If you haven't already shot your footage, you can still try what I described, just to see how simple it really is. Just put an image on track one, then a PIP object on track two. (They should be the same length.)

Kevin

I see you're calling this solved now. What did you end up doing?

I did exactly what Kevin64 stated and it worked great. Very, very easy. I kind of thought about doing this as I would with the crossover, but wasn't sure how to have the subject fade away. You saved the day. Thanks so much.

Glad it worked out. Some people (not you, clearly) are intimidated by keyframes and seem hesitant to mess around with them even when it is explained to them that it is the way to do what they want to do. They are a very useful tool.

Kevin