What is the reason for the patach under the vuv in וַיִּקְרָא, the first word of the third book of the Pentateuch?
I would expect a schva, which, at least in modern Hebrew, is what sits under a vuv that means "and," like this.
This is the standard "Vav ha-hipuch" of Biblical grammar, which reverses the future tense to the past.
"Yikra" == "he will call."
"Ve-yikra" == "and he will call."
"VA-yikra" == "[and] he called."
(It's actually unclear whether the vav hahipuch also functions as an "and." Most translators include the "and", though Rabbi Aryeh Kaplan's Living Torah doesn't.)