What is a vendor when talking about java?

Vendor means implementer of JVM, is JVM/JRE by Oracle (or) IBM (or) Some other. Each of these have their own implementation of JVM.

Example:

java.vendor = Sun Microsystems Inc. 
java.vendor.url = http://java.sun.com/ 

You can simply fire command from command prompt to get java.vendor detail

java -XshowSettings:properties -version

or you may refer below url for more info on Java properties

https://www.applicationproductionsupport.com/2019/05/how-to-get-java-vendor-information-and.html

A JVM can be developed by different companies if it follows the Java Virtual Machine Specification. You could even make your own implementation if you wanted to read through the specification. The "Oracle Corporation" that you saw shows the Oracle developed your current JVM. Different companies make their own JVM's if they aren't satisfied with the Oracle one. For example, a custom JVM should still be able to run Java code correctly but it might offer some advanced security.

Vendor is just the creator/maintainer of the JVM. Sun was bought by Oracle a couple of years ago so the "Sun JVM" more or less got relabeled as the "Oracle JVM". There are a lot of different JVM implementations. Each one has to follow the JVM Spec. Take a look at the full List of JVMs