I have a VPS and a separate space accessible with FTP.
In case of problems I can only recreate the starting image of the VPS. I have no other tools, if not particularly expensive.
Being a VPS I don't have USB ports, tape or burners available.
I therefore have to make a backup via FTP and keep it updated automatically.
In case of problems I should just restore the starting image provided by the ISP and then restore everything I have saved at a given time of a certain day.
Which means backing up the ENTIRE server, including open files.

If then the backup also allows me to recover the single file or the single folder it would be the best.

What do you advise me to do?

Since only you know which ISP you use, and what type of backup this ISP offers, only you can find out what backup options exist. I would use the ISP's documentation or support.

A few general considerations follow.

You may be able to snapshot your server. Depending on the way your ISP implements them, snapshots may not be as safe as backups, but they are a good solution to go back to an earlier version of your filesystem in case you accidentally remove or otherwise damage files. You may also be able to mount a snapshot and just restore single files. You may be able to create several snapshots, e.g. one per day, and keep a certain number of them.

Alternatively, just create a (ideally compressed) tar archive of all your files and store it in that FTP space. You can then recover single files from such an archive without much effort. If your data is very critical, however, the FTP space is problematic, since you are unlikely to know where it is located. Are you sure that this FTP space is available in case the VPS's storage system breaks? If not, store the tar archive in several locations that are unrelated to each other. In that context, I suggest you learn about the 3-2-1 backup strategy (3 copies of your data in 2 formats [disk and tape], 1 copy offsite).

I may not have been clear, but I intended to backup from Linux, not using the ISP tools there are, but too expensive for my use of this VPS.

The FTP service is external to the VPS and has nothing to do with it.
So there are no problems of availability.
I can also add that if I close the relationship with this ISP I could use this backup to configure a VPS at another ISP, obviously always starting from a Linux image of the same type: Debian 10 with Debian 10 or Ubuntu 20.04 with Ubuntu 20.04, etc.

I can't adopt the 3.2.1 strategy because with a VPS I don't have two different formats: tape and discs.
I would like to adopt a classic Grandfather-Father-Son backup strategy, but saving the backups to the three different remote destinations, but always using the ftp/sft channel.

It is obvious that such a strategy, as well as 3.2.1, cannot be done by making backups by hand.
This assumes a backup software that at certain times of the day does the Full/Incremental backup depending on the day.
And if the software managed deduplication it would be ideal, to save space and backup time.

I was asking for advice to find the right tool.
Software like Bacula, for example, doesn't seem like the right tool for a VPS.

I find my vps is very reliable and I have never lost anything unless I screwed it up myself. I have scripts I keep that will enable me to recreate any of my vps's at any time. I usually do experiments on a local virtual box so I do not mess up my production server and I use rsync from a cron job for backups of anything hard to recreate like my source repository.

I agree with the above comment, I haven't faced any issues with the VPS I have, and having scripts to recreate any point is the best and most secure way to go. I use the Linux VPS server from a company that one of my friends recommended and I'm really impressed with the services overall. I think that's also to take into consideration when choosing what host you want, and it relates directly to how much you have to stress about securing your data and preparing in case of server failure. Because I use servers I find it even more important to look at reviews and ratings than you would normally do when looking to buy a PC or a laptop.