Are there simple tweaks that I can visual select everything with in a pair of fold markers? Just like what
vi" will do for a pair of double quotes. Preferably
vi plus some other keys.
What you are looking for is called a "text-object" in Vim terminology and you can find the list of these text-objects at
:h text-objects. As you can see there is not a built-in text-object for the folds.
However you might be interested in this plugin: vim-textobj-user. It allows you to define your own text-objects easily.
Note that if you don't want to use a plugin you can simply use an operator pending mode mapping (see