I've never eaten Lender's Bagels so I have no idea if they are vegan or not. However, that customer service rep is an idiot. Just because something is kosher does not mean it is vegan. Foods containing animal products/flesh can be kosher (not pork or shellfish) as long as the animal was slaughtered in accordance with "proper guidelines" and certain products (meat and dairy) are not mixed. Kosher does not imply vegan... though it doesn't rule it out either. This letter tells you NOTHING.
The person who replied to you is an idiot. However, I have been in contact with a rep from Lender's who *has* a brain, and was assured that all suspicious ingredients were not animal derived. My guy did his homework, and I trust him.