Linux

Veracrypt has back-up headers located elsewhere in the volume that are unlikely to have been overwritten.

First thing's first I would strongly recommend copying the drive as it currently exists bit for bit to another drive of equal or larger size. Don't work on the original if you can help it.

Now with this copy, you should try to check the option to use the backup header when mounting and try again. If the partition is gone and veracrypt doesn't see it you'll need to try using something that recovers partitions and doesn't mind encrypted partitions or partitions or file system types it doesn't understand and use that to ON THE COPY recover and recreate the partition (this will write data and can cause the possibility of further loss or worsen your ability to recover which is why it is important to perform it on a copy). Testdesk may work for this but there are other options that probably are better.

See this list: https://old.reddit.com/r/datarecovery/wiki/software and choose something from there if this data is truly important. Again only work on a copy on another drive. Some of these software examples actually work against the original drive and make a copy elsewhere and should be safe to use on the original drive so long as they have you select a target drive to push the recovered data to but read the documentation. Testdisk absolutely must be used on a copy.

You will incur data loss and likely have to run a file system checker on the drive once successfully mounted.

I guess it's a question of how much hassle it's worth. I did a messy data recovery of a crashed database for a work client once, but it involved a lot of trial and error and writing special purpose code, plus considerable luck that some things worked better than I had a right to expect. Cost of something like that would be in the multi kilobucks, maybe low 5 figures. We got almost all the data back, though not 100%.

Maybe just put that HDD aside and replace it with a new one, and deal slowly with recovering the data as you get the time to mess with it. Also don't do any write operations on the old drive. Maybe copy it entirely to someplace and work on the copy. In fact better do that anyway, HD's physically crash all the time.

VeraCrypt Volume Format Specification:

Each VeraCrypt volume contains an embedded backup header, located at the end of the volume (see above). The header backup is not a copy of the volume header because it is encrypted with a different header key derived using a different salt (see the section Header Key Derivation, Salt, and Iteration Count).

It may be possible to recover the encryption key. You might try asking on VeraCrypt forums/mailing lists or contacting a commercial data recovery service which understands VeraCrypt. Though I’m not familiar with VeraCrypt so I may be misunderstanding the cited documentation.

My condolences. That data is now gone, I suggest you square yourself with that and move on. Save yourself a lot of grief and time.

If you have your encryption key backed up, you have a chance to decrypt it still. It's also possible, but unlikely, the key somehow survived the ISO write and it was written elsewhere on the drive, allowing the key to be recovered. I would only trust such with a professional. (There is basically a smaller encrypted section that your typed-in password decrypts, that section contains the encryption key the rest of the drive uses.)

Honestly though, if you have your stuff backed up (you do have your stuff backed up elsewhere?!?), just restore from your backup and call this a loss.

If you don't have a backup, this was your wakeup call. Always have a backup going forward.

If it wasnt encrypted you could have used testdisk app but I dont see how you could decrypt it in this state

I'm gonna be the one to say it. You've ruined your ability to decrypt the data. You can try a recovery service but expect to pay a lot for zero results.

I'm sorry this happened to you.

Edit: don't go with commercial software, find a recovery service

First thing it did was overwrite the partition table and everything else with that, to make its own, since it could disregard all the existing data.

I agree with the other commenter, commercial recovery, if the data was that crucial.

I think you need to go commercial recovery. If it was a file you accidentally deleted, that can easily be recovered, but you wrote directly to the device.