The eTrex Vista Cx has the same antenna as the Garmin Legend, which I have a lot of experience with. I also have a Garmin 76C and the Garmin 76Cx. (The 60C, 60CS, 76C, and 76CS share the same processor.) (The 60Cx, 60CSx, 76Cx, and 76CSx share the same processor.)
The Garmin “x” series share the “x” designator because they have a removable memory chip. But the Legend Cx and Vista Cx do not share the same processor with the 60 Cx, CSx, 76Cx, and 76CSx. The highly sensitive receiver in the 60Cx and 76Cx series makes a major difference. I have done some testing in heavy oak canopy.
One of the tests involved creating Tracks using the Garmin Legend, Garmin 76C, and the Garmin 76Cx. The test included hiking down an over grown mining trail which had little sky showing, and taking a 90 degree turn into vegetation so heavy that the ticks carry chainsaws. When I returned home I downloaded the Tracks created on each unit to MapSource, US Topo on my computer. It was clear that the reception obtained by the 76Cx far exceeded the other two units. In fact the Legend and the 76C where ineffective in this heavy cover while moving. Locks could be obtained in some location by sitting still.
More recently I mapped tracks created by the 76C and 76Cx on a three-mile hike and a four-mile hike. On these tests, the canopy was not as thick as the original test. But the printed track from the 76Cx was complete, and the track from the 76C was about 95 percent of the 76Cx track.
Inside my residence I can get a satellite lock with the 76Cx, there are select locations that I can get a satellite lock with the 76C and Legend. An external antenna is more important with the 76C when navigating in a vehicle. The Garmin Legend Cx and Vista Cx do not offer an external antenna connector.
When hiking a trail in a State Park I had to climb up the side of a hill to an opening to get a lock using the Garmin Legend. I believe Vista Cx would require the same side trip.
Owning a Garmin Legend and a Garmin 76c I spent the addition funds for the 76Cx because it is that much better.