How to make vindicator open doors

This sounds like a perfect fit for sculk sensors, but they won't be available until 1.17 is released.

Without them, there are still a few options:

• Pressure plates, which you already said would look ugly.
• Tripwire hooks, probably uglier unless you can hide them in the walls or with some kind of decoration.
• Redstone ore glows for a while when you step on it, and an observer would be able to detect the change, both when it starts glowing and again when it stops. It would look uglier than pressure plates... however, you can cover it with carpets and it still works¹, making it the least intrusive option of them.

That's the only options I can think of, at least in Bedrock, survival mode. In Java edition, you could also build a much more difficult, but completely hidden, pufferfish detector. But as usual, those don't work in Bedrock edition.

_{1: Tested on Bedrock edition 1.16.210.}