In Vegas Pro, are you using "Open copy in Sound Forge"? which imports the selected event's audio data to Sound Forge and names it 'Take 2', VP places it on top of the original event, (the T key toggles the takes)
OTOH, "Open in Sound Forge" opens the entire media file for (destructive) editing. If you cannot overwrite the file with the same name, it is likely open in another application (Vegas Pro in this case).
Open Vegas Pro and select the "Options> Preferences> General" menu. Enable (check-mark) the item "Close media files when not the active application".
This should allow saving (overwriting) the file in Sound Forge.

btw, This is actually a VP issue and maybe should have been posted in the VP audio forum, but it is related, so no harm/no foul.
In any case, welcome to the Magix Sound Forge users forum @Frederick-Dispensiere8136.

"Close media files when not the active application" is checked. I use "Open copy in Sound Forge." I then tried "Open in Sound Forge" also. Only workaround I can find is Open Copy, then save with another file name. Then go back to Vegas, delete the original sound clip, and drag the new one into its place. It used to work automatically with no problem. I have just uninstalled/reinstalled Vegas Pro 18 - same problem. Next I will un/reinstall SF 15. Thanks for all your replies. I welcome any other suggestion(s).

I cannot duplicate that behavior with any version of Sound Forge Pro (10 thru 15) along with Vegas Pro16. It may have something to do with VP-18 which I do not have.
What format is the audio files? If your audio media is something other than PCM, the only setting I can think of is in Sound Forge, "Options> Preferences> General> Always proxy compressed formats"

I have SFP15 and VP18 on my PC. Open in Sound Forge from VP18 is working fine for me.

I just ran a quick test. I right clicked on a selected event in VP18, selected Open in Sound Forge. SFP 15 launched with the event portion highlighted, I adjust the volume of the waveform, clicked saved and witnessed changes take place in Vegas.

I tested using an .MP3 file open in Vegas. I have the Always Proxy compressed formats unchecked in Forge preferences.

Maybe try running Vegas and/or Sound Forge as administrator on your system?

Another thing I would try if you haven't done so already. In both Vegas and Forge, they always default their temp working directories and recorded files to folders on your C:\ drive. I've been finding Windows to be finicky with apps saving to certain folders on the C: drive. I have all my working directory folders set to my non OS D: drive.

That's the reason I suggest maybe running Forge and Vegas as administrator may help.

I have been able to cure the problem with not being able to overwrite the Vegas audio track after editing in Sound Forge. After OPEN COPY IN SOUND FORGE, once file (track) is loaded, I have to minimize Vegas window. Then I can work the file in SF and save it back to Vegas. One issue still remains - if I leave SF open and then go to Vegas to another clip/track and OPEN COPY IN SOUND FORGE - it opens another instance of SF. That never happened with earlier version(s) of Vegas and/or SF. Thanks to everyone for their advice! Greatly appreciated.

@Frederick-Dispensiere8136
Maybe you should also read this article in the Vegas Pro forum for information
https://www.vegascreativesoftware.info/us/forum/open-copy-in-sound-forge-gives-2-different-problems--128997/#ca802070