What to say.... the smoldering emotions between Jack and Irina are much more romantic, seductive and hot than the Syd/Vaughn thing IMHO, but that is probably because I'm older. And uhm, as someone else stated, some things get better over time!
I think the fact that Jack and Irina have so much history, love, hurt, betrayal and etc its more deep and it is awesome to see the flames of desire and love still burning there. I think that they both still have it for each other. Bad.
Syd and Vauhgn's love is new and exciting and stuff, I'll grant you that, but the backstory on Jack and Irina makes for some great deep emotion and tension.
I'm holding out that either Irina and Jack are in on something together here to bring down Sloane, or that Jack knows Irina is acting on her own to bring down Sloane and condones her actions here for Syd's sake, or that IF Irina is totally playing Jack that he knows it and is playing her right back.
I'm a hopeless Romantic at the mercy of JJ and company.