What is the single most valuable hard skill a recent college graduate can learn to maximize their career potential and salary?

Negotiating starting salary/raises is the SINGLE most important skill for maximizing salary. Right out of college the difference between taking the offer and skillfully asking for more is often ~30%. Since raises are usually a percentage, this becomes even bigger thanks to the miracle of compound interest. Not to mention that knowing how to ask will get you more/better raises.

All of the languages you listed are useful. The better question is what languages should you learn to achieve the salary you want. That you already know VBA is good, because that means you can learn programming. But trust me when I say you won't make real money doing that. Automating tasks for people doesn't grow revenue/profits, it just minimizes trivial costs (at best). Java, Python and/or R are great ways to get paid, because you'll be able to work with all forms and quantities of data. There's nothing wrong with web design languages (PHP, JS, HTML), but scientific programming/math often pays quite a bit since the decisions being made using data are crucial to company success.

It depends a great deal on several factors: where you want to work the industry in which you want to work your capabilities how much you can invest (time and money) To be blunt, the word Peoplesoft flashed into my mind as a hook to follow. But I have no idea how popular that is nowadays. Better you should research some interesting companies and jobs within them, then see what hard skills they require. As in any project, start with the goals and work backwards.

Here's an article on 5 computer programs that employers want you to know. being proficient with these programs gives you a huge leg up on competition http://www.collegeworks.com/blog/5-computer-programs-every-employer-wants-you-to-know/