Hmm, like I said xenophobia doesn't make a lot of sense since it was the aliens helping that Earth recovered as fast as it did from WWIII.
But I can see the crew being xenophobic could be a plot element of the internal feuding between the Xenophobic "Stay to Earth" guys and the more open-minded ones: Once the Government decided to approve the deeper exploration missions the Xenophobes pulled strings to make sure some of the ships would be crewed by their own choices, with the potential hope being that their exploits would either result in anything they find being for Earth's advantage and Earth alone, or negative results which they could use to shut down the Deep-Space mission altogether.
I'd say go with the Vulcans, because the Minbari are too isolationist at that point. The Vulcans can be a "failed experiment" of the Vorlons: They taught the violent but powerful Vulcan people the way of logic and control and banished the original Vulcans who disagreed with this (the Romulans) meaning the Romulans are closer to "True" Vulcans than present-day Vulcans. But the Vulcans' couldn't be totally brought under control as Vorlon Vassals like the Minbari so they withdrew their support and left them be. The Minbari and Vulcans have a sort of "estranged brothers" relationship as a result.
The Andorians should be the toughest military guys on the block, perhaps the Narn are in a junior alliance with them (the Narn are weak partners) and still have their Cold War with the Vulcans.
This would be the Vulcan impetus for helping Earth, they don't want them to become Andorian Vassals as well.
How to fit the Romulans into the shared Vulcan/Minbari history...hmmm.