I haven´t been able to fix vsync in Jazz Jackrabbit. Everything else works perfectly, but there´s an annoying tearing most times there´s a vertical scroll (it´s noticeable just by loading any level and looking down pressing the down button). I had the old original version running in Dosbox, and I had the same problem. (Happy buyer anyway because I didn´t have all the extra episodes or the CD version).

I played this game back in the day, but I can´t remember if it had the same problem or not. Is it a problem of the game or is it fixable in any way? Has anybody managed to apply vsync in this game? I´ve tried Dosbox ECE SVN with no luck either.

The screentearing is a shortcoming of dosbox at the moment.
There are a few threads over at Vogons addressing this issue and to my knowledge there aren't any fixes yet (but it never hurts to take a look or try something yourself).
Best hope is they fix this in a new build, if there ever comes a new one.

I've managed to apply Vsync through Intel HD (graphics properties). Not sure if this is possible with Nvidia/Radeon though. The bad news is, while the tearing is gone during the gameplay, Vsync with this game adds some more problems: longer freezes between transitions such as loading screens, start of level etc. Not to mention the main menu stutters badly (including menu music severely stuttering) and selecting things in menus is very sluggish.

All these pauses are caused by dosbox 0.74; try to use dosbox svn daum or dosbox ece

Thanks KMetalMind, those settings fixed everything completely, I'm pleased to say! Thanks too to all the other patient people who helped along the way, leading to this - and other - improvements as well as solutions. It really seems like different setups were sort of working for different people, with few saying that any method solved everything without compromise. This one, however, fixed it 100%. Cheers!

*Edit*. I've just noticed the image is slightly squashed horizontally. If you look at the sun on the very first stage it looks squashed horizontally / stretched vertically / whichever way you want to look at it (it looks wrong).

To avoid a stretched or squashed image on a 16:9 display, you could use:

fullresolution=original
output=surface
aspect=true

If you then switch your display to 4:3 mode it should look right and the sun won't look like an egg :P (well, oval).

The only downside is that in 4:3 there will black borders all around and the whole image is smaller on-screen. I personally don't mind that too much because the tradeoff is a more natural picture (less blown up and pixelated), but that's a matter of personal taste.

note: 16:9 looks horrible when using output=surface of course, with a squashed-looking picture.

The game runs at 16:10, not 4:3.

Some of the menus and such are 4:3, but the game itself is 16:10.

By "4:3" I just meant switching my widescreen monitor to 4:3, otherwise it looks all squashed. The surface setup I mentioned introduces borders top and bottom that aren't present with the direct3d way, as well as the obligatory borders to the sides. What you're left with is a kind of widescreen view (16:10?) and everything no longer looks distorted.

Whether the game was supposed to look like that I'm not sure, but it looks better to my eyes - all down to personal taste of course.

I'm curious, are you saying that the game wasn't designed to look that way? In other words, for the authentic experience, which way should I be displaying it? Tall? Squashed? Or natural (round sun and ammo pickups)?

I tried this guide here on these forums that stated I could simply switch out 0.74 with the SVN Daum version

But when I used the shortcut, all it would do is open DosBox, and then close, the guide doesn't work, and I don't think it's as easy as "replacing the DOSBox as the guide implies. How can I change the version to the SVN one and have the shortcut work? Because it doesn't.

This is what worked for me:

1. In the game's folder, I deleted the *contents* of the DOSBOX folder. If you want, you could keep them as backup and put them somewhere else or whatever, but I just deleted them.

2. Place the contents of the newly extracted SVN Daum folder in the game's (currently empty) DOSBOX folder.

3. Inside the DOSBOX folder of the game, look for the SVN Daum CONF file, called simply 'dosbox'. Rename it 'dosbox_jazz' and then move it out of the DOSBOX sub-folder, placing it in the game's folder (typically located at: C:\GOG Games\JazzJackrabbit) so that it replaces the original dosbox_jazz CONF file.*

*Alternatively, you may want to keep a backup the original DOSBox CONF file for this game, in which case I suppose you could rename it and put it somewhere else if need be (again, I just deleted the old stuff to keep things simple, but you may want to back it up).

No need of DOSBox with this game.
To fix this game on Windows 10 1903 for best smoothness and to run directly the Jazz2.exe, just set-up Microsoft Application Compatibility Toolkit and NVIDIA Profile Inspector like this (see attached):