Hello,
My nextcloud instance is hosted on a VPS and if I configure (with the External Storages app) an SFTP server on my home network using BitVise SSH Server, will the files uploaded by users be saved on the PC’s hard drive that I have set up the SFTP server or in my VPS storage?
If you upload stuff in this external folder, it will be uploaded on your home network sftp. If you haven’t fast up- and download that can be tricky (timeouts during slow up-/downloads).
I have successfully set up an SFTP server. So I’ve got another question. Is it possible to force all user files to be uploaded on my SFTP server and not the VPS storage space? If so, how?
I think you can minimize the cloud storage perhaps to 0 bytes 
But why do you not install a nextcloud in your home network with DynDNS and port forwarding for 80/443 and Lets Encrypt. I think it is much faster and no problem because of security. A lot of self hoster use it.
Debian and Apache2 (Nextcloud, MariaDB, Lets Encrypt)
https://www.howtoforge.com/tutorial/how-to-install-nextcloud-on-debian-10
Ubuntu an Nginx (Nextcloud, MariaDB, Lets Encrypt)
https://www.howtoforge.com/tutorial/ubuntu-nginx-nextcloud/
You can combine it like Debian with Nginx or Ubuntu with Apache2.