Lance wouldn't think that such a brief encounter could make Tamara lean towards him or be wholeheartedly devoted. He's not narcissistic to that extent; on the contrary, he has a clear understanding.
First of all, Tamara has a history and a rather vile personality. She had to rely on him previously to gain leverage in the organization, but once she's established, things might change.
After all, the Romu Organization's higher-ups can't really allow Tamara to favor an outsider; they'll surely work on her, and it's possible she might abandon him for resources or other benefits.
Though Lance felt a bit regretful about this, gaining Amanda in return was a great deal, as she's far more reliable in Hamlet than Tamara ever was.
"I find it very hard to contact her."
Grendel knew there was a problem when she saw Lance fall silent, and her face turned nasty when she heard this.
