You can easily split panes (called windows in vim) within vim. You can also create new tabs within vim for related things you need to do.
I still use tmux or screen to do other things which aren't convenient or elegant to do within vim, but splitting windows and creating tabs can be done just fine within vim itself (or, in my case, within gvim, which I prefer due to the increased color gamut and keybinding abilities over terminal vim).
I still use tmux or screen to do other things which aren't convenient or elegant to do within vim, but splitting windows and creating tabs can be done just fine within vim itself (or, in my case, within gvim, which I prefer due to the increased color gamut and keybinding abilities over terminal vim).