Which Buddha do Buddhists take refuge in? Does one take refuge into the concept of Buddhahood or refuge into Gautama Buddha?  What about Vairocana Buddha or Amitābha Buddha for that matter? I've heard various things from taking refuge in a cosmic Buddha principal to the actual Gautama Buddha over the years from various Buddhist teachers.   Perhaps taking refuge into the Buddha is a relative subjective experience.  Which Buddha do Buddhists take refuge in?
I think both interpretations should be somewhat intertwined and doesn't need to be clearly distinguished. See, you'll not say 'I only take refuge in Gautama Buddha, but not Buddhahood', or 'I will take refuge only in Buddhahood, but not Gautama Buddha', both of these sounds weird. Buddha Gautama represents Buddhahood, so we can not separate Him from Buddhahood.

Abhidharmakosa-bhasya of Vasubandhu explains taking refuge in Buddha like this:


  They who take refuge in the Buddha, take refuge in the factors
  (dharma) of the perfected being (arhat) that constitute a Buddha
  (buddhakāraka), the factors that are the causes of the designation
  “Buddha” [buddhaprajñaptihetūn], i.e., the factors due to which, as a
  principal cause, a certain person is called Buddha; or else the
  factors through the acquisition of which a certain person,
  understanding all entities, is called Buddha. These factors are (1)
  the cognition of exhaustion (kṣayajñāna), (2) the cognition of
  non-arising (anutpādajñāna), and (4) right view (samyagdṛṣṭi) (vi. 50,
  67), (4) with the factors that accompany (parivāra) these cognitions
  (jñāna), i.e., with the five pure aggregates (skandha). 
  
  As for the material body (rūpakāya) of the Buddha, it is not
  subject to modification through the acquisition of the status of
  Buddha. Thus, one does not take refuge in the material body of the
  Buddha, which is, in fact, the material body of the Bodhisattva.
  
  [Question:] – Does one take refuge in all the Buddhas or in one
  Buddha? [Answer:] – According to the nature of entities [lakṣaṇatas],
  and in the absence of an explicit declaration (kaṇṭhokti), [one takes
  refuge] in all the Buddhas, for the Buddhas have always followed the
  same path, (1) a mundane (laukika) path and (2) a supramundane
  (lokottara) path (vii. 34).  [Lodrö Sangpo translation.]