One thing they told me, and that wasn't on their webpage as far as I could see, is that if you have more than one overstay during the last 3 years your application for Elite visa will be denied. Can be good to know.
I was denied tourist visa there a few weeks ago and got no stamp or anything in my passport, and was even denied to know the reason why I was denied a visa. But I had two tourist visas already in 2019. Went back to Thailand with bus and no problem at all with visa exemption at Nong Khai.
No one can know, but reading about peoples experiences in Vientiane there is a clear risk that they will deny you a visa.
No one seems to have had problems crossing by land at Nong khai though, so even if you're denied a visa at the embassy you can get a visa exemption 30 days. But if I was you I would rather try another embassy than the one in Vientiane.
I am there right now and can add some info if you plan to come here.
They only allow 130 visa applicants each day. I arrived 09.15 (they open 09.30) and there were probably 70 people already in line. The line kept growing behind me so get here early if you want to be sure to be let inside!
They also do not provide any forms or copyservice so be sure to have all your documents ready before getting here.