The Ceiling of the Vault is shaped in a heptagram to represent the union of all Grades up to the Abyss. Centered in the middle is the Triangle of the Third Supernal Order. The names of the Sephiroth are written in Hebrew in the seven inner triangles of the heptagram.  Their corresponding astrological symbol is attached to the right of the name - in all instances but one. The sign of Mercury is to the right of נצח, while the sign of Venus is to the right of הוד.  It is strange that these are switched. At first I considered this to be a mistake of the transcriber, but William Westcott has this to say:

Now these planetary sides are found to be in a special order, neither astronomical nor astrological. The common order of the succession of the planets is that defined by their relative distances from Earth, putting the sun, however, in Earth’s place in the series thus: Saturn, Jupiter, Mars, sun, Venus, Mercury, moon. Saturn is farthest from the Earth, and the Earth is between Mars and Venus. Beginning with Saturn in the case of the walls of the Vault, in the order is Saturn, Jupiter, Mars, sun, Mercury, Venus, moon. Here Mercury and Venus are transposed.

He goes on to explain that the organization is instead based on the light spectrum, the colors from the prism, rather than their celestial order:

Red - Mars
Orange - Sun
Yellow - Mercury
Green - Venus
Blue - Moon
Indigo - Saturn
Violet - Jupiter

Mars and Jupiter point to the east together because of their attributions on the edge of the spectrum, Red to Mars, and Violet to Jupiter, lending to Infrared and Ultraviolet. These elements of life that exist “beyond direct experience” represent the manifestations of the Third Order, which is nonterrestrial and nontemporal. Westcott phrases it this way:

These forces, ever present and unseen, are represented by the Chief Adept standing erect at the eastern angle, the most powerful person in the group, and delegate of the chiefs of the Second Order, and through them of the mystic Third Order... bringing intuition with him.

Venus’s displacement has another meaning. Venus is in the west, that of the Evening Star, together with the Setting Sun. This contains a lot of symbolism regarding resurrection, a theme that is also in the Masonic Master Mason degree. Venus is also a symbol of completion, being the only planetary symbol that can contain the entirety of the Tree of Life. 
This is also a symbol of the unity of the planets, contruent with the heptagram in more ways than one, as seven is also the number of the sphere of Netzach. It is also quite fitting as the candidate of the 5°=6 ° degree of the Golden Dawn enters under Venus, as Daleth means “Door.” Not just any door, “The Bar of Heaven,” providing a door to the Supernal Triad,  within which is the Rose.   It is also interesting that the previous grade relates to Netzach, and also contains the Grade Symbol of the Philosophus Degree: 4°=7°, both numbers of Venus, with four being the number of Daleth, the corresponding Hebrew letter. The Philosophus Degree is also attributed to Fire, an element of Growth, and as what Westcott mentions, Intuition.