Vivitar 28mm f2.8 Macro lens on my K50..need some help

You mentioned "seems to work in P too", plus the fact that the lens has an "A" setting, suggest that the camera can control the aperture. You can confirm that by setting "Av" (aperture priority) mode - with the lens aperture at "A", you should see the aperture comes alive on the display, and when you turn the dial (the one on the back of the camera, not the one in front of the shutter release) the aperture on the display should change. "P" (program) mode works as well because the camera recognizes aperture settings, but it will be set to the widest aperture of the lens. It is much better to use the "Av" mode when you control the aperture and let the camera determine the shutter speed / ISO. Of course "M" mode always works.

NOTE: With a manual lens, only "center-weighed" and "spot" metering modes can be used.

As for manual focusing - you should switch the "AF/MF" switch on the left side next to the lens mount to "MF" first.

If you use Live View, you can use Focus Peaking to help with focusing. If you are not familiar with this, you should refer to your user manual. When you use the viewfinder to focus, the camera will quickly flash the red dot when you are in focus. This may somewhat help you in dim light when it is difficult to see if the subject is in focus. Another more drastic way to improve manual focusing is to buy a split focusing screen and swap out the original screen of the camera.

A better way I discover, however, is to use the "catch-in-focus" function of the camera. Please refer to your user manual for detailed procedure but I am going to outline it below:

1. activate the "catch-in'focus" function (Menu -> Custom menu ("C") -> page 4, option 22 "Catch-in-focus" -> "On" -> OK

2. set "AF/MF" to "AF" (yes it sounds strange for manual focusing but this is required)

3. put the lens grossly out of focus, then slowly turn the focusing ring to start to bring your subject into focus

4. keep pressing the shutter release button while you continue to slowly move into focus

5. as soon as the subject is in focus, the camera will automatically fire the shot as long as your finger is pressing the shutter release button

Once you understand the sequence of the procedure, you can do this much quicker. I find that "catch-in'focus" works best in good lighting, and may not be accurate in dim light.

I hope this is a start for you to further explore the camera's functions.