My husband and I are volunteers for Habitat for Humanity in Vietnam in March. We have a visa approval number for our volunteer work dated from 18th March - 16th April. However we now arrive in Vietnam as tourists on 10th March. Is it ok to get tourist visas from 10th March - 17th March only (not a 30 day visa) as we have been told the visa dates must not cross over. We leave Vietnam 4th April. Thank you for your advice.