bnussbaum, you'll find tons of reading here on the forum on this issue.
Those bike rack receivers you can buy at the dealer and clamp onto the bumper will result in the bikes bouncing as you drive down the road and exert a twisting force of the thin metal bumper. That twisting force is likely to pull the entire bumper away from the frame its attached to. Even 2 bikes is a big risk. If you know a welder, you could have them weld a hitch receiver to the Vibe's frame so that it will be well supported to resist the forces the bouncing bikes will exert.
I have enough space in the bed of my truck so I just put them there. If that's not an option for you, do some searching here and you'll find lots of ideas.
2011 Rockwood Signature Ultra Lite 8293SS, 12K Equal-i-zer
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73
2010 Rockwood Roo 23SS (2012 - 2014)
2009 Ford F-150 XLT SuperCrew, 5.4L, 4x4, 5'6" bed (2012 - 2016)