First of all, go into your preferences and tick Yes Where it says Always allow HTML. Then click save. (Preferences located here)
First of all, simply choose a category and make a new post.
Once on that new post, write some text, or simply find the vine you want on http://vine.co.
Now once you've found the vine you want to post, click the embed button to the left of the page.
Once you're on the embed tab, click the code at the bottom. Just click it then click copy.
Now copied, you can paste the link into the post box.
Make sure before posting that you untick the Disable HTML in this post option.
Once unticked, press the Send button and you're ready to go!