What do I fill infusion slots with?
Im a Zerker DPS Guardian and just got full ascended gear. I have 40 laurels and all my gear has empty infusion slots. I was looking at the laurel vender and saw they have pretty lame upgrades like +4 vit/power/etc. Are those the best things to slot? If I infuse all the items, do I get a bonus Agony Res slot or does that Defensive slot turn into the AR slot? Or can I have both?
You need to build infusions that give you AR if you want that. You can also use Laurels and Badges of Honor to buy infusions that give you bonuses against guards in WvW. See:
http://wiki.guildwars2.com/wiki/Infusion
Also, is the +20 MF laurel upgrade best for amulet?
That depends on our priorities. That’s what I run, but you really want to get your MF over 100 if you can to see a real difference in drops.