The optimal parameters for memcache on a VPS?

Universal rules no setup.
You just need to understand what these numbers:
MAXCONN is approximately the number of simultaneous connections that the server will work.
CACHESIZE - the amount of space in RAM in MB.

So find out how much free RAM you have, estimate how much you can allocate for memcached and write.
Somehow, I doubt that you are on a VPS 50GB of RAM, so before you write 50000 figure out this point.

Just installed and configured the memcached server itself doesn't cache.
Caching is performed inside the application.
Here and there you will see(set the cache read the cache) if it works, then works "normally")

1.

The parameters MAXCONN, CACHESIZE and OPTIONS are selected experimentally depending on the nature of the load and available resources.

To estimate the amount of memory needed for caching (CACHESIZE option), the size of your file cache. If you have the project file cache is 3 GB, the use of memcached c 256MB memory will not be effective due to frequent displacement.

Taken from here: https://dev.1c-bitrix.ru/learning/course/?COURSE_I...

If all the memory to give memcached, then the rest will be nothing left.

2. For "to make sure that the" use https://github.com/lagged/memcache.php
This script shows the use of a reserved memory cache. Ie periodically monitors the condition, it is possible to estimate, add, or conversely, to remove the amount of memory allocated for memcached