Sorry that you missed it. The brewery is always a nasty place to try and get actual cans due to the tourism and such. You will have much more luck locating a local store in the area that The Alchemist distributes to.
Recently, I believe, they have started doing staggered distribution around the VT area meaning that just because one place is out on Friday doesn't mean everyone one is empty.
The Beverage Warehouse in Winooski, VT (maybe 25 minutes) from the brewery does staggered releases of HT. I was visiting my parents a couple weekends ago after the VT Brewfest (AWESOME!!!!!!!!!) and stopped by BW on my way out of town on Sunday. I was able to pick up 2 4-packs and a couple bombers of Lawson's (Chinooker'd and Session IPA).
It took a little waiting around but they do a good job of letting you know when you can get your hands on it. They are also a lot more receptive than the brewery staff. I love The Alchemist and all that they do but the times I have called them about acquiring HT, they have been blunt and rather harsh about how I will most likely go home empty handed. I am sure it is just because they are constantly having to deflect those calls.
If you check out their webpage they give a list of local stores that carry it. Won't help you now as you have left, but next time you are in the area you can plot out like 4-5 beer stores to hit and at least one should be able to help you out. Still is a hard one to find though... even for a VT native haha.