Well `$sinSigma` can be negative. I also think `0.0` is more readable :)

Can you add the falling test ? Something like:
```php
public function testVincentyDistanceCoIncidentPoint()
    {
        $ellipsoid = Ellipsoid::createFromName(Ellipsoid::WGS84);
        $this->distance->setFrom($this->getMockCoordinateReturns($this->coordA, $ellipsoid));
        $this->distance->setTo($this->getMockCoordinateReturns($this->coordA, $ellipsoid));

        $this->assertEquals(0.0, $this->distance->vincenty());
    }
```
in `tests/League/Geotools/Tests/Distance/DistanceTest.php` ?