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As at 9th April 2019 you will not need a visa in the UK, although it is possible that things might change after Brexit (now 12th April, although a delay is possible)
As a Spanish citizen you can enter Vietnam visa-free if your stay is less than 15 days. (Source)
If you intend to stay more than 15 days, or if you intend to return to Vietnam less than a month after your visa-free visit you'll need an eVisa, applied for online and collected on arrival, cost from $17 (Source as above).
Notes: