When I compare pair programming in person vs. by video chat, there is no comparison. In person is always more effective. That said, if you have no choice (your pairing partner is not in the same office), then video chat is way better than 10 years ago, and is a reasonable alternative.
As someone who has poorer vision than average, pair programming in person as the observer is almost impossible. Id have to ask the driver constantly for clarification on what they typed or have them increase their text size to something that would be uncomfortable to them.
You raise an interesting and fair point. I "raise you" (poker like) with this challenge: What if you are sitting next to each other with side by side PCs? Your PC can have gigantic "boomer-style" fonts (no hate on that; each year over 40 for me... I swear I need a bigger font!) and your (twenty-something whipper snapper!) teammate can have microscopic 8-point fonts. Do you think the communication will be better face-to-face or over video? I still stand by my original point: Human communication is always better face-to-face compared to video.