Anything you salt will firm up in texture over a period of time. I suspect that since you cooked these with seasonings and then chilled and left them in the fridge before reheating an eating they firmed up a great deal in the fridge.
If you check out this blind tasting conducted by Dave Arnold at Cooking Issues you'll find some more detailed info about this topic. The gist of it is that for cook-chill-reheat purposes you shouldn't salt the meat before searing. For cook-direct serve meals your just fine doing it that way.
http://www.cookingissues.com/2011/10/12/to-salt-or-not-to-salt-thats-the-searing-question/
The machine
I invited a couple of friends who are keen to try sous vide over during the whole process. All 4 of us agreed. The machine is going back.
My immediate first thought was that you had a bad machine. The Vac-Star Sous Vide Chef has excellent reviews on Vac-Star's website and it looks like you are using it within proper specifications. That would be my first hunch, as nothing else seems particularly out of place. If you are able to come back, since this was over a year ago, it would be curious to see if a different machine solved your problems.
On a side note, I looked up reviews for Sous Vide Supreme vacuum pouches and they seem good, too.
Other things
I looked up for reviews of the I noticed the Beyond Salmon blogger had posted a follow-up experiment with sous-vide steaks that I found had useful data. This summarized it well:
What most sous-vide books and websites tell you is that you can't overcook using the sous-vide method. It all depends on how you define "overcook." The steak will not go above the desired temperature no matter how long you hold it in the water bath (well, dah!), but the longer you hold it, the more juice you lose.
I have noticed this from your post:
I have tried various temperatures and times from 55°C to 60°C and from 40 minutes to 4 hours.
Per her experiment, steak A, B and C turned out the best. They also had the lowest temperatures and cooking times:
Steak A: 121°F (49.5°C) water bath for 50 minutes
Steak B: 126°F (52.2°C) water bath for 50 minutes
Steak C: 131°F (55°C) water bath for 100 minutes
It likely would be a good idea to try lower temperatures, and stick to the lower-end of the time scale. Also from the article:
But if you are using an immersion circulator, you might be tempted to put meat in the water bath whenever it's convenient (in the morning before going to work, during kids' nap, etc.) and then have it ready for dinner. It will surely be very tender, but you'd better have a lot of demi-glace handy because it will be dry.
Hope this helps!
Best Answer
Yes, it can both spoil and be unhealthy, even deadly. The micro organisms and toxins they can produce were there before you put it the bag and vacuum sealed it to cook it sous vide. If the temperature you cooked it at was not high enough to kill the micro organisms and destroy their toxins, they are still there. Just vacuum packing it a second time then leaving the package at room temperature has changed nothing. The food can still spoil and still make you sick or kill those who consume it. Leaving it at room temperature has allowed them to multiply/grow.