If water vapor is always blown away into space, how is it able to create chemical compounds on Venus?
Think about the water constantly being both added to and removed from the atmosphere.
Adding water:

Water migrating from deeper layers of the planet crust and mantle into the atmosphere.
Water being formed from sun wind hydrogen and the available oxygen in the atmosphere (the high D/H ratio hints that the primordial water dominates on Venus)
Water being formed from hydrogen and oxygen produced by decomposition of other substances in the atmosphere.

Removing water:

Photolysis - water decomposed to hydrogen and oxygen, both of them engaged in other chemical processes
Escaping hydrogen and possibly whole water molecules into the space.

There is always some equilibrium between the available water and hydrogen and also other hydrogen containing substances like sulphuric acid (that's what Venus clouds are made of) and the widely-discussed today phosphine.

On Earth, we have stratospheric and mesospheric clouds made of water, sulphuric acid and nitric acid in atmospheric layers that are otherwise notoriously dry.  On Venus, the atmosphere has little free oxygen so it is tolerant to much more of a chemical diversity. (E.g. phosphine would self-ignite in Earth's oxygen-rich atmosphere)