There used to be feature in the task manager to “bring to front” max runing proces… but thats gone, at least for me, probably some awesome windows update.

SOLUTION 1 :
Go to TaskManager, in process tab go to 3ds max application. Expand and you will see V-Ray frame buffer, right click and BRING T O FRONT.

SOLUTION 2 :
Go to TaskManager, in process tab go to 3ds max application. Expand and you will see V-Ray frame buffer, right click and BRING T O FRONT. Try using “Windows + Left/Right Arrow” that will force the window to change between screens. Then you can press “Windows + Up Arrow” and it will maximize the window.