When I select a block of text using the mouse, Vim goes into

selection mode instead of Visual mode. Why? The ‘selectmode’ option controls whether Select mode will be started when selecting a block of text using the mouse. To start Visual mode when selecting text using mouse, remove the ‘mouse’ value from the ‘selectmode’ option:

    :set selectmode-=mouse

Note that by default, the ‘selectmode’ option will be set to empty, so that always visual mode is used.