The heirlooms on the guild vendor are just like other heirlooms in the game except that these must be unlocked via a guild achievement and you must have the required guild faction to purchase them. After that, you can use them just like normal heirloom items.

We did have thoughts of making these items guild bound a long time ago but we came to the conclusion that that was just too limiting of a design. Imagine leaving a guild and losing all the guild rewards that you worked so hard to get? Taking items away from players is something that we would like to avoid at all costs.