I'm on doxycycline tablets I've taken 2 but the second one made me vommit I'm meant to be on them for 7 days. Does that mean that it is ineffective?

Still effective. Missing one or two doses won't make any difference in the effectiveness of doxycycline for most infections. Anyway, if vomiting was more than an hour after taking the drug, enough was absorbed into your system to be effective. Taking the rest of your doses with food will reduce the chance of further nausea and vomiting.