Also wondering about this because one of my references said that VMCAS wanted her to attach a letter, but it did not say anything about it being on official letterhead so she just typed up a letter and attached it. On the old format the person doing the reference was able to just type in their comments at the bottom so no letterhead necessary. Definitely concerned about this now!
Individual LORs are done electronically. If it's a committee or a composite letter, there's no requirement for letterhead, but they must be signed by each person that contributes. Composite letters are signed individually and grouped together, committee letters are signed by all the people on the committee. In both cases, an online evaluation through VMCAS has to accompany the uploaded letters. If it's a composite letter, each person who signs the letter has to complete the online evaluation form. If it's a committee letter, then the committee chair has to complete the online evaluation form.