Can I extend a single entry visa at Immigration in Bangkok for thirty days? Is Savannakhet generally lenient on new visas?

Depends on what kind of visa you have. Only a tourist visa can be extended by 30 days, for 1,900 baht a copy of passport and arrival card and passport photo and a form that needs to be filled in.

If onle a few tourist visas, normally you will get a few till you have about 5 of them. Might even get a double entry tourist visa.

Closed as the OP already asked his question here: http://www.thaivisa.com/forum/topic/618656-limits-on-thai-visas-from-vientiane-1-or-2-days-for-thai-visa-from-cambodia/#entry6106931