For many accelerators, virtual interviews fall into two categories. They are either a screening vehicle for the face to face interview or a replacement for the face to face interview; especially for remote startups. Where a virtual interview is used to shortlist accelerator applicants for the face to face interview or for the next round of interviews, it will usually be much shorter (e.g. between 10 - 20 minutes). Often it may be conducted by accelerator operational staff, and not the program managers, who ask 2 - 5 questions to determine if the founders are real, what their story is, their level of commitment /maturity and their stage of evolution or development.
Interviewers may also ask indirect questions checking for the presence of any red flags. For some accelerators you may have two short virtual interviews (e.g. each with a different team or each having a different focus). Either way, at the end, the interviewer (s) will usually make an on the spot decision and let you know whether you are advancing to the next interview stage - although sometimes this decision may come within a week of the virtual interview.
The first virtual interview could go longer and be held by a more senior and final decision maker (e.g. a partner or program manager). Whether this happens or not usually depends on how your startup came to submit the accelerator application. For example, if you were targeted and invited by the accelerator to submit an application, you may be deemed to have already passed the first interview screening. Not all startups do a virtual interview (e.g if founders live locally then a short face to face interview can replace the virtual interview). Founders living locally may still be offered a virtual interview; but those who choose a virtual interview may signal a lesser level of commitment to those that choose a face to face interview.