You can vote on a suggestion in two ways:
Firstly, you can do a "quick vote" from the All Posts list by clicking on the vote button, highlighted below. This will also show you if you've previously voted on a suggestion as the arrow will be green. To remove your vote, click the button again, and the arrow will return to it's greyed-out state. You are unable to down-vote suggestions, only remove your vote.
The second way of voting is to click into the feature suggestion. This way you can also read through any discussion before making your choice. The voting button is still located next to the title.
In the above screenshot you can also see how commenting works. You can both add a new comment, or reply to a particular comment. You can also sort comments with the newest or oldest showing at the top. Using the @ symbol, you can highlight a person directly even if they haven't entered the conversation yet.