Even though this information is about B (i.e., tourist and business) visas to the US, it may offer some insights into trends related to the issuance/refusal rates for student visas. You can assume that, in most cases, the refusal rates are higher. Below are the B visa refusal rates for the top ten sending countries for US-bound international studens, using SEVIS (Department of Homeland Security) data from March 2012:
China: 12%
S. Korea: 7.5%
India: 30.1%
Saudi Arabia: 6.3%
Canada: 52.2%
Japan: 18.7%
Taiwan: 1.9%
Vietnam: 33.5%
Mexico: 12.8%
Brazil: 3.8%
Aside from Canada, whose high refusal rate is explained below, and India, whose population is 12 times larger than Vietnam’s (i.e., sheer numbers), Vietnam has the highest refusal rate. Here are the rates from 2006-10:
2010: 36.1%
2009: 42.3%
2008: 38.8%
2007: 36.3%
2006: 40.9%
The average among all countries except Canada is 14%. The likely reason why Canada’s refusal rate is so high is that there are a lot non-Canadians applying for visas at the US diplomatic posts in Canada. (Canadians don’t need a visit to travel to the US.)
This information for every country whose citizens visited the US as tourists or on business is available on the US State Department’s website (PDF).
Stay tuned for another post about Vietnam-US immigration. Can you guess what place Vietnam ranked in this category last year? My post will provide information from a 10-year period; the results may surprise you.
An International Educator in Viet Nam 
the refusal rates are related to nationality, not residence or location of application, so this does not really explain Canada. Further, the population of India cannot explain its refusal rate without also making a comparison with China which has an even greater population size but a significantly lower refusal rate.
Thanks for your comment. Understood re your first point, hence my clarification about Canada’s high refusal rate. There are many factors that influence a US Mission’s issuance/denial rates, including a particular country’s track record (e.g., fraud, use of non-immigrant visas for the purpose of immigration, etc.), the policy of the current ambassador, US foreign policy du jour, etc.