No visas are required for a business or vacation stay of 90 days or less. Residents of some countries will need to pay a fee upon entering the country, in particular if you’re from a country that charges a fee for Argentines to visit your country, as it is more of a tag you’re it type of thing rather than wanted bureaucracy. But this can be taken care of at the airport upon arrival, and you should check with your consulate or a travel forum for what the exact fee is at present.
No vaccinations are required. The free, socialized medical system is accessible by anyone including tourists and should an emergency arise you will be treated regardless of your insurance status and rarely charged for anything other than medications. Yes, we realize that for most Americans this concept of actually providing free health care to people who need it may be a foreign concept, but rest assured it is a reality in Argentina. There are some private medical systems that offer a cost effective alternative to the public system, and in both cases you can purchase valid in-country insurance fairly easily either online or by calling in advance. Read more about that here.
Check with your cell phone provider about coverage. The alternative is to rent a local phone, or purchase an inexpensive one and purchase a chip and phone credits. Speaking a bit of Spanish will help, especially outside of Buenos Aires, but is not absolutely necessary as you will pick phrases up whether you want to or not.
Cash is the preferred payment method in most places throughout the country, although the major tourist destinations cater to credit and debit cards and traveler’s checks. Either way you’ll soon find yourself pulling out cash from ATMs frequently, so check with your bank about fees and possibly set up an alternate account in advance in order to minimize the fees, and also see if your bank will reimburse you for them.