I'm hesitating to jump into this discussion because I may only make things more confusing. Marco has explained the reason for Quantize to Frames perfectly. We give you the option to turn it off and cut as freely as you want against the best advice.
The reason to be very careful cutting with Quantize to Frames turned off is that the result of your cut will be unpredictable and could result in a bad edit. When you cut between two video frames, you don't know whether that cut will be interpreted during render to happen at the frame before the cut or the frame after the cut. As Marco points out, there is no "in between frames" cut in the final video. The cut has to be pushed to one frame or the other in the final render. So, in short, when you cut video between frames you simply don't know what you're really going to get, and you could get an unexpected hole in your video or some other (unpredictable) problem.
Now, OP has been happy with his results cutting that way, and if that's the case, keep doing what you're doing. It's just better for you to do it with the full information that Marco and others have given. Your results are unpredictable, but it's up to you to decide to live with that unpredictability or not.
And in the end, the thing you initially posted as a problem is not a problem. VEGAS always shows you the frame you are on in the timeline. If Quantize is off and you are between frames, VEGAS considers you (rightly) to be on the current frame (which you interpret as the previous frame, but it is not because a frame lasts for the duration of time between its beginning frame boundary and the next frame boundary). This simply will not change because the behavior you're asking for is incorrect behavior where VEGAS would be showing you something other than the frame you're actually on.
Good news: you are free to edit just as you want! Bad news (for OP): VEGAS will continue to show the right thing (the current frame), not the next frame.
Finally, just for the record, if you are making an audio-only edit (that is, you're editing just an audio event) VEGAS automatically ignores Quantize to Frames and lets you edit freely. So, even with Quantize to Frames on, you can hold your Shift key as you trim the audio event. The Shift key temporarily overrides grouping and you are now able to edit the audio event to any location in between video frames.