It's a somewhat long shot, but if I were you, I'd give it a try again, using another form of vanilla (maybe precook a pod in the milk, then scrape out the seeds and add them), no corn syrup at all, and pay attention to using sweet butter, not cultured butter.
Fudge is all about forming the right size crystals in the supersaturated sugar solution. From your description, no crystals form at all. This could happen if you inadverently invert your sugar (= split it into glucose and fructose). In home cooking, this is usually done on purpose with acid when making non-crystalized candy. So, just in case that either your butter or your vanilla extract contains some acid, I'd use the versions guaranteed not to have any, vanilla pods and sweet butter. It's not sure that they are doing something wrong, because butter in the supermarket is rarely cultured, and vanilla extract doesn't contain acid normally, but it's better to make sure.
On an industrial level, invert sugar is created with enzymes. While corn syrup starts out as starch and not sucrose, I can imagine their enzymes being able to split the fructose off the sucrose in your sugar too. I don't know what they do to the syrup exactly, and maybe the heat will deactivate the enzymes anyway, but I think it is worth a try to make it with pure sugar, so there is no source for enzymes to land in your fudge and invert it.
If none of these is the culprit, then I'm afraid the next likely explanation is the mechanics of the beating. But maybe it's not about the hand vs. mixer beating as such. It's possible that you've been stopping too early, or that changing the attachment used for beating will change the outcome.