Planetary phase angle of Hohmann transfer
Let's assume that Earth and Venus have circular, coplanar orbits.
The spacecraft advances 180&deg; in heliocentric ecliptic longitude in 143 days; meanwhile, Venus advances 241&deg; and Earth advances 141&deg;.
When the spacecraft departs from Earth (left), Venus should be 61&deg; behind Earth as seen from the Sun.
When the spacecraft arrives at Venus (right), Venus should be 39&deg; ahead of Earth.



The Sun-Earth-Venus angle marked in green in your second figure is Venus's elongation, 43&deg; east of the Sun as seen from Earth at departure time.

In another context, "phase angle" could refer to the Sun-Venus-Earth angle.
At departure time, an Earth-based observer would see Venus in a 76&deg; phase angle or 62% illuminated.