Here's a trick you can try as well - I've used this method without seeing any performance hits (fps drops) in the game. I'm still using an older NVidia GTX 1070 btw. This trick works because while SDVX in windowed mode doesn't allow the window to be resized (I've tried several different "force window resize" programs), it does allow the window to be *maximized* (double-click the title bar).
Obviously if you maximize the SDVX window in a 16:9 landscape screen area, you'll end up with a bizarre massively stretched out screen that looks all wrong. So what you do is set a custom resolution in NVidia that actually has a 9:16 ratio in landscape mode. This will provide a portrait aspect screen on your monitor with black bars on each side that still takes advantage of the full vertical resolution of your monitor (1440p in your case).
So for your 1440p monitor, you' set a custom resolution of 810 x 1440 . Then run SDVX in windowed mode. Once the window pops up, double-click the title bar and the game window will fill up the entire screen area. The aspect ratio will be very slightly "squished" because the title bar is still there, but this will be pretty much unnoticeable to human eyes. You can tweak the horizontal resolution by reducing it by a few pixels if you want an "exact" 9:16 aspect ration in the gameplay screen.
I know this isn't a true "fullscreen" solution, but I find that this method works quite well & I haven't encountered perfromace issues or fps drops.
I'll mess around & see if I can get it to run in full screen at a custom resolution & follow this post up if I find a way. I've always just run SDVX in windowed mode up until know as I've never had any issues with it.