What kind of server to chose on VPNs ?

[ambipro]

I don't really understand what you mean about "what IP listed in the swarm" how do I know this IP ? how do I control it ?
and also for the https, both questions ... is it to say that I shouldn't use an UDP VPN server ? because it doesn't support https (?)
"if you use https trackers only and Full-Stream" > I don't know what it is, both

This is the IP that Deluge announces for others to connect to, if you want to check, use https://ipleak.net or https://www.whatismyip.net/tools/torrent-ip-checker/ and do the "activate" on torrent option.

But I understood that the encryption could be easily better by the VPN than by the client, but how can I know if my Vpn provider uses an encryption ? and of which level ?

Ask your VPN provider, or check the website. It should tell you. Whether you set it up to use this is up to your configuration.

I ve noticed that if it had a good level of encryption, I could disable the client encryption and have a better connectivity

Which to chose in Deluge ? handshake, fullstream or either ?

If you are concerned about privacy, more encryption does not hurt you assuming you're PC is not slow.

Full stream > Handshake > Disabled.

From my understanding, using the client encryption will require the peer you are uploading/downloading to/from have at least this level enabled on their client.

[Menard]

Ask your VPN provider, or check the website. It should tell you. Whether you set it up to use this is up to your configuration.

From my understanding, using the client encryption will require the peer you are uploading/downloading to/from have at least this level enabled on their client.

No, they tell nothing about this, but they have special server for torrents, and we don't know what they have different. Speeds are the same.
I could ask but I guess the lambda team doesn't know it either.
And for all questions like can you open a port etc as well

OK for your last sentence that informs me more about how it works