A web developer is not a type of organization, keep the organization itemtype to just describe the organisation offering the job. For the job itself the closest vocabulary on data-vocabulary.org would appear to be Offer but I would say that's a bit of stretch semantically.
However, I don't think you'll see much immediate benefit to marking up with a non-standard vocabulary so I would recommend confining your microdata to those parts that can be marked up (eg. company contact information) rather than trying to add microdata to everything.
There is a job-listing brainstorming page on the microformats wiki but it hasn't seen much recent activity, you could try posting on the mailing list to see if anyone's interested in taking it forward. If that gets some traction there may be some benefit to using that vocabulary.