There is a possible workaround for this situation, I have done the following:
Write the credentials into a text file, named somename.txt.
Copy or move this file to some system folder, that can not be accessed by error while normal use of the telephone.
In the settings of VPN you can set 'import credentials from file…" and give a location where this file is.
Enter the location where you copied the file with the credentials.
After this, the VPN fetches the credentials from this file and you don’t have to input them manually.
Since I did so, I never again had to re-enter my VPN credentials (since many months)
The text file should contain absolutely nothing except the credentials!
1st line: username[Enter]
2nd line: password
Nothing else should be in the file. This works. The only disadvantage is that your credentials are in an unencrypted file somewhere in the system.