All embassies and consulates now use the online evisa system
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You have to be in the country where you’re applying from. Vientiane uses a hybrid sort of system where you apply online and when you’ve applied they contact you to tell you to go to the embassy to pay.
Think you may be thinking of malaria. Very uncommon. Dengue is not. Many many cases of it. It’s incredibly horrible to get and will have you bedridden for days on end.
In theory there are vaccines for it but I have no idea if they’re effective
All offices will accept a walk in application for an extension. Depending on which office you use you may find making an appointment saves you hours in a queue.