I accidentally replace vanilla Sequin Dress with a mod. Is there any way to get it back without reinstalling the game?

If you installed the mod via NMM, simply deactivating it in the NMM interface should already restore the original file. NMM never deletes or overwrites existing files without backing them up first.

If that is not the case, you - somehow - overwrote the existing file permanently, resulting in it being lost. However, if you run the game on steam, you can check the gamefiles via properties. If something is missing, steam will redownload it, that should fix your issue.

If you're not on steam, you might be out of luck, I'm not sure if the bethesda client offers such a feature. In that case you can only reinstall it. I would not recommend to try and fix the issue manually by copy-pasting the game file from somewhere. That will only lead to even more issues.