The first amendment reads: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances."
What that clearly says is that the federal government can't establish a state religion. It can't suppress religion. It doesn't say anything about the government being restricted from supporting ALL religions which it does now my making them tax-exempt. Education vouchers that allow a taxpayer's money to go to any school, religious or secular ISN'T supporting any particular religion. So it isn't violating the 1st Amendment's actual language or intent.