I found a super simple way to restore VitaShell!

1) Install QCMA - https://codestation.github.io/qcma/

2) Install PSVIMGTOOLS-FRONTEND - https://bitbucket.org/SilicaAndPina/psvimgtools-frontend/downloads/

3) Plug in your PSVita via USB to your computer. Run QCMA, then open PSVIMGTOOLS-FRONTEND. (Your account and AID should be auto detected.)

4) On PSVIMGTOOLS-FRONTEND's popup menu, press "Easy Installers" > "VitaShell" > "Install" (button). The program should put Vitashell in to your default CMA "APPS" folder on your computer.

5) Open Content Manager on your PSVita. Press "Copy Content" > "PC -> PS Vita System" > "Applications" > "PS Vita" > "VitaShell" (checkbox) > Copy.

DONE! Now you have VitaShell back on your device!