Some recent eyewitness experience about getting from Van to Orumiyeh via the secondary border post at Sero: Both Best Van Tur and Van Golu coach lines offer services. Either will sell you a ticket for their own service (20 YTL), but only the service scheduled to depart Van first is likely to run as the later service may not have enough passengers. When I went through, Best Van had a 9 AM and Van Golu a 9:30 AM service. The Best Van service departed, the Van Golu one was cancelled - leaving me standing on the peron. I recommend talking to both companies before buying your ticket!
The border was the smoothest, most hassle-free crossing of my trip. Helped by the fact that there were so few of us to process, we were through into Iran in less than 45 minutes. There is no free-for-all open range between the borders: you enter a building at Turkish immigration and exit it after Iranian immigration - the two areas are separated only by a short corridor. Beware the exchange sharks on the Iranian side. I suspect The Reza Hotel in Orumiyeh charged me the full double room rate, but it's always possible that the single room is not available.
Check cheap Iran tours.