Vack made his acting debut in the short film Dear Diary (1996), which won the Academy Award for Best Live Action Short Film. Vack has guest-starred in single episodes of Hope & Faith (2004), Third Watch (2004), Law & Order: Special Victims Unit (2005), Ghost Whisperer (2009), and Cold Case (2010).
From 2011 until its cancellation the following year, he portrayed Jason Strider in the short-lived MTV television series I Just Want My Pants Back. Since 2014, he has starred as Alex Merriweather in the Amazon Video comedy-drama series Mozart in the Jungle, alongside Malcolm McDowell, Lola Kirke and Bernadette Peters.
