Fastest way to Samadhi I have heard that Metta Meditation is the fastest way to a samadhi mind. How effectively fast is it? How is Metta properly cultivated as a meditation? As in how to develop the Vitakka Vicara on metta in meditation?
I have heard that Metta Meditation is the fastest way to a samadhi mind. How effectively fast is it? 


It depends on the practitioner. All beings have different Kamma. Some have sharp and balanced faculties and others must work more diligently on theirs.

The initial effort/goal of any (Samatha)meditator must be to quiet down the five hindrances. They are an obstruction to Samatha meditation. As long as the hindrances are present strong concentration (Jhana) cannot arise.

The two most influential hindrances are 1) sense-desire and 2) ill-will. The Buddha often talked about how ones faculties cannot remain sharp if one is prone to indulge in sense-desires, e.g. in MN 75 - Magandiya Sutta:


  "So too, Magandiya, in the past sensual pleasures were painful to
  touch, hot, and scorching; in the future sensual pleasures will be painful
  to touch, hot, and scorching; and now at present sensual pleasures are
  painful to touch, hot, and scorching. But these people who are not free
  from lust for sensual pleasures, who are devoured by craving for sensual
  pleasures, who burn with the fever of sensual pleasures, have faculties
  that are impaired; thus, though sensual pleasures are actually painful
  to touch, they acquire a mistaken perception of them as pleasant."


So the fastest way in any form of Samatha meditation is to ensure that the hindrances are kept in check. 

This can be done in a number of ways; most importantly is to guard/restrain the sense-doors. 

That can be done either by practicing


Morality (precepts)
Mindfulness (guarding the sense-doors, Right Mindfulness)
Attaining Wisdom (into the 3 sign of existence)
Patience and Forbearance (Khanti)
Effort (Right Effort from the Noble 8-fold Path). 


If just one of these methods arises then one has fulfilled the goal of restraint. 


  How is Metta properly cultivated as a meditation?


See my in-depth and detailed answer on how to pratice Metta Meditation correctly.


  As in how to develop the Vitakka Vicara on metta in meditation? 


Vitakka and Vicara is practiced in the same way as in all other forms of Samatha meditation.

Vitakka means to apply the Mind to the object and Vicara means to continually keep the Mind on the object. 

Vitakka is like striking a bell once.

Vicara is like striking a bell continually with a little pause between strikes so that the bell sounds will have a balanced and continuous flow.

Hope this helps. If you have any questions to the post feel free to ask.