Personally, if your budget allows I'd go for an audio interface other than the mbox. M-Audio interfaces are ok and many people are happy with them, but I've also read complaints about M-Audio interfaces over the years. If your budget allows check out Presonus or MOTU or Apogee or RME.
A firewire interface will give you better results if you need to record multiple audio tracks simultaneously - if not then USB should be fine.
Logic cannot read VSTs. There are "wrappers" that can convert some VSTs to AUs and allow Logic to read them, e.g. check out FXpansion.... but it's better to just install the AU version.
The vast majority of 3rd party plugins include the AU and VST versions in the same box. All the major ones are in VST, AU, RTAS, etc formats. When you install these you'll either get a choice or it will automatically install the AU and VST files to your Mac.
Hope that helps