The tool can be found on its github.
The Releases session of the page contains the latest version of the tool.
Be sure to read how to use before trying to make use of it.
https://github.com/nakano15/VSSaveManager

The first window will ask you to pick a folder and a user profile to load.
If you have Steam installed in another place other than the default C: directory folder, click Browse and select the folder Steam is installed.



The list above the Open button should be populated with lists of random number. Click Open, and it will try loading a save file on that profile.
If the save file loaded doesn't match the status of your last playthrough (character and stage selected), or the tool say there's no save file in that directory, reopen the program and try another profile.

The tool itself doesn't have a working hashcode maker, so when you save a new SaveData modified by the program, or create a empty save file, you'll need to use a Save Editor for Vampire Survivors to update the hashcode.
You should be able to find online tools that allows that.

Be sure to update the hashcode everytime the program generates a SaveData.
The SaveData the program creates will be found on your Desktop, so better clean it up a bit if it's a mess. This is no longer necessary because the program can now update the hashcode of the save file checksum.

If you're unsure wether it will work or not, backup your save file before doing the change.

It will create a save file similar to the one generated when you first start playing the game, with the diference being the broken hashcode.
That's perfect if you want to restart the whole game from 0.

If that's what you want to do, skip to "I modified my save file or created my empty save file, what now?".
Or, if you want an alternative way of restarting your progress, check bellow.

New Game Plus is another way you can reset your progress, but with a few little benefits or changes for your future playthrough.
Yes, that means you'll start all the way from Mad Forest too, and have to unlock mostly everything again, depending on what you pick on the settings.
It offers you the following benefits.

1. You can pick any of the characters you previously unlocked, to have it already unlocked once you start a new playthrough.
   
2. You can choose to have Arcanas unlocked right from the start, which also gives you Sarabande of Healing.
   
3. You can pick to have Game Killer already unlocked once you start the new playthrough. If you have Arcanas unlocked, that means you can have a whole Level 1 playthrough, if you dare.
   
4. You can have some relics unlocked if you want, like Grim Grimoire, Ars Gouda, Milky Way Map, Magic Banger, Mindbender and Forbidden Scrolls of Morbane. Or, you can only pick to unlock only the relics that doesn't affect progress, like Magic Banger and Mind Bender. Or pick to not receive relics at all from game start.
   
5. You can set all starter characters but Antonio to be locked, and need to be bought again.

To unlock the possibility of starting a New Game Plus, you must have beaten The Directer on your current save file.

Now, the SaveData made will be found on your Desktop. Use a Vampire Survivors Save Manager to update the hashcode of the save data, and then save the changed SaveData somewhere.
Be sure to leave the file named "SaveData". Ready for placing on save folder.

The program will have already opened a folder where your Vampire Survivors save files are located.
If you want, backup the SaveData in it, in case you fear something go wrong (simply copying the file or renaming it will do).
Then, move the modified SaveData you also updated the hashcode of, and place inside the folder where your save files are at.

Before running the game, disable Steam Cloud Backup, or else it will revert the save file changes upon opening the game.

Now, all you need to do is open the game and see if it will run.
If no errors happen, then that means you did it correctly.
If a error happen saying something about length or something, is because you didn't used the save file with a changed Hashcode. You can use a Vampire Survivors Save Editor to update the hash code of the save file. And also report that happened.


An example of a working modified save file loaded in-game.

Well, I hope this tool help you guys extending your playthroughs on this amazing game, and also incentivate making new runs on it with different characters.

Thanks to Takacomic for fixing the issue impeding the program from generating the correct checksum hashcode.

Have fun :)