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Spain is a Schengen treaty country, so you will need a Schengen visa:
Holders of a Vietnamese passport require a visa for the "Schengen States".
http://eeas.europa.eu/delegations/vietnam/eu_travel/visa/index_en.htm
The cost for all temporary visas is 60 EUR, and you also need insurance and a lot of documentation. If your summer school regularly takes in foreign visitors, they should already know what visa you need and how to apply for it, so talk to them first.