VPS, that is, virtual private server, is a virtualization system that instantly allocates to you what you use.
For example, let the ram given to you be 8GB. Let’s say another server is 12GB. The main server is 16GB in total. In this case, the 12GB server will use the RAM that you are not currently using. When both servers need to run at full capacity, 2GB will be restricted from both VPS as the main server is missing 4GB. Thus, you will only be able to use 6GB until the other server starts using 8GB instead of 12GB. In summary; You will not get the service you paid for!
In VDS, that is, Virtual dedicated server system, all the resources defined for you belong to you only. If you have an 8GB VDS, you can use it whenever you want independently of other VDSs. As long as your server is running, this amount of RAM is blocked on the main server and reserved for your VDS. So you can use resources instantly and fully. So you get the service you pay for!