To minimize this effect on an HDTV, use settings that "soften" the picture (or make it less sharp, or reduce the frequency response.) Using "Movie" mode will help with this. Avoid modes called "Dynamic" or"Vivid" or "Sports", which will make your VHS movie look worse (noise artifacts more visible.) You can also turn down "Sharpness" in your picture settings. (Movie mode does this for you.) Also look in your menu for picture settings such as "Digital Clean View" or"Noise Reduction". Turning these on may help.

Here is why:
The problem is that an HDTV has a native resolution that about twice that of your standard definition VCR, both horizontally and vertically. So, in other words, the HDTV is displaying about 4 times as much information (or 4 times as many "dots") on the screen as your VCR is providing. The TV has to create information to fill in between the "dots" to be able to fill the screen.
Different HDTVs have different methods for creating this extra information. The simplest method is interpolation - basically creating something that is an "average" of what is on either side - or "connecting the dots." Some TV manufacturers have more sophisticated algorithms to determine what to create to fill in. For very sharp, high-quality standard definition source material (such as from a DVD), these can work well - and yield a picture that looks almost HD. But, if the source material is low resolution or noisy, this does not necessarily give a good picture. The noise can trick the TV into filling in something that doesn't look good. Pre-recorded movies on VHS may not be too bad, but something recorded from poor quality analog cable TV will be noisy, and if it was recorded at a slow speed (4, 6, or 8 hour speed) the quality will be even worse.

Other suggestions:
Assuming that your VHS movies are 4:3 aspect ratio (not widescreen), you might try setting your TV to display them in 4:3 (black bars on the side.) Otherwise, it will stretch them horizontally to fill the screen, which can make things look worse.

Also, your picture will be better (more detail, less noisy) if you connect the VCR to the TV using the video & audio (yellow, white, and red) connectors rather than the RF (cable TV type connector on channel 3 or 4.)

Even with the suggestions above, it still may not look ideal, but it should at least look somewhat better than it would otherwise. I hope this is helpful.