Vertical blinds can be pesky, they just hang there and can be quite unimpressive. It can also be hard to get anything to stay on top of them.
If you buy garland, and it needs to be the harder tree-like kind, you can shape it to sit across the top. The stiffness of the garland will allow it to sit on top without slipping off, and you can form the branches into more authentic looking greenery. Then, you can string lights across the top. The garland will provide a great resting place for lights and won’t require tape or anything to hold them up!!
See the pictures below for examples.