Let's say you have the following picture...

App

--> Dependency A --> Dependency C (at version 1.2.3)

--> Dependency B --> Dependency C (at version 1.3.4)

The latest release of Dependency C is 1.4.5. Which version would be used after running `vgo get -u`?

As you note it would use the newer version of those specified (version 1.3.4). The newer releases after those explicitly specified are not used.