He was just excited for whatever made her happy. That was how Lavender ought to be - happy. Zach knew he had his priorities right. If something or someone besides him made someone happy, then that was what was good for them. And Zach loved people, and so he wanted them to be happy. It was okay if it wasn't him. And Ven, Zach knew Ven loved Seamus and was in love with him and all that. It's not like he ever expected her to choose him over Shay for raising the bun with.
Zach gave her a huge, not too tight but a big one. "Oh, that's awesome!" Zach exclaimed, "I'm so happy for you! That's just what you wanted."