Would the Monk’s robe give the monk give the stat bonus of treating the character as a monk or not if the character has monk as a secondary class. The monk’s robe reads as:
“When worn, this simple brown robe confers great ability in unarmed combat. If the wearer has levels in monk, her AC and unarmed damage is treated as a monk of five levels higher. If donned by a character with the Stunning Fist feat, the robe lets her make one additional stunning attack per day. If the character is not a monk, she gains the AC and unarmed damage of a 5th-Level monk (although she does not add her Wisdom bonus to her AC). This AC bonus functions just like the monk’s AC bonus.”
And the secondary rules state:
“Variant Multiclassing
This optional system allows a character to trade out half her feats in order to gain the benefits of a secondary class. These rules enable characters to gain many of the benefits of multiclassing without sacrificing advancement in their primary classes, and creates opportunities to explore novel character concepts, such as a barbarian whose rage stems from being afflicted by the gods with an oracle’s curse and revelations.
Under the standard rules, multiclassing can lead to a wide disparity in character ability. With this system, each character can choose a secondary class at 1st level that she trains in throughout her career, without giving up levels in her primary class. Once selected, this choice is permanent (though if using the retraining rules, the secondary class can be retrained by paying half the cost of retraining all her class levels). A character who selects this option doesn’t gain feats at 3rd, 7th, 11th, 15th, and 19th levels, but instead gains class features from her secondary class as described on Table: Multiclass Character Advancement. It is probably a good idea to use either this variant system or normal multiclassing, but it’s possible for the two systems to be used together. In a game using both systems, a character can’t take levels in the secondary class she gains from this variant.”
With the monk option stating:
“A character who chooses monk as his secondary class gains the following secondary class features.
Armor: At 1st level, he loses all his secondary monk abilities when wearing armor, using a shield, or carrying a medium or heavy load.
Unarmed Strike: At 3rd level, he gains the Improved Unarmed Strike feat and the unarmed damage of a monk of his character level – 2.
Evasion: At 7th level, he gains evasion.
Ki Pool: At 11th level, he gains the ki pool class feature of a monk of his character level – 2, with a number of ki points equal to 1/2 his character level. He only ever gains ki pool (lawful) if he is of lawful alignment.
AC Bonus: At 15th level, he gains a +3 dodge bonus to AC.
Improved Evasion: At 19th level, he gains improved evasion.”
So with rules as written is the Variant Multiclassing considered levels in monk to qualify for the monk bonus on said magic item?
You don't have levels in Monk, so only the second part of the description applies. That means you get a +1 AC bonus. You do technically gain 1d8 unarmed strike damage, but unless you have a 13k gp item ptior to 6th level, your Unarmed Strike class feature does that anyway (and of course raises your damage to 1d10 at 10th level as normal).
So yeah, +1 AC for 13k. Since the AC bonus class featuere is not actualy taken from the Monk, and grants a dodge bonus rather than an untyped bonus, I'd say the robe would stack with the 15th level ability.
Definitely not worth it then.