They are responsible up to a point, generally until the point they drop your well packaged and stealthy order into the mail system. After that if customs get it you may be able to split the loss 50/50. When it comes to vendors using tracking to prove delivery, if they didn't they would have heaps more buyers trying it on saying they never received their order. Personally I do not think I have ever permanently lost an item of mail, stuff doesn't just go missing so for it not to arrive something went wrong, wrong shipping details, shitty packaging causing it to get sprung, something like that.