VMware does not support this for 7.0 due to USB wear. For a lab, it works with tweaks.
Since there is no "portable" installer in the traditional sense, you "make" it portable by installing the OS onto a secondary USB stick rather than a hard drive. Step A: Prepare the Bootable Installer Download or Etcher . Insert a temporary USB drive (the "Installer"). Select your downloaded ESXi 7.0 ISO and flash it. Step B: The "Portable" Installation esxi 70 download iso portable
If the "portable" idea is too complex or unsupported, consider these alternatives: VMware does not support this for 7