Rama's promise to Sugreeva

वालिनो मे भयार्तस्य सर्वलोकाभयंकर
ममापि त्वमनाथस्य प्रसादं कर्तुमर्हसि
एवमुक्तस्तु तेजस्वी धर्मज्ञो धर्मवत्सलः
प्रत्युवाच स काकुत्स्थः सुग्रीवं प्रहसन्निव
उपकारफलं मित्रमपकारो अरिलक्षणं
अद्यैव तं हनिष्यामि तव भार्यापहारिणं

vAlino me BayArtasya sarvalokABayaMkara

mamApi tvamanAthasya prasAdaM kartumarhasi

evamuktastu tejasvI dharmaj~jo dharmavatsalaH

pratyuvAca sa kAkutsthaH sugrIvaM prahasanniva upakAraPalaM mitramapakAro arilakShaNaM adyaiva taM haniShyAmi tava BAryApahAriNaM  

Shloka Translation:

Frightened Sugreeva sought with Rama asking that Vali is a Terror to the world and since you too are an orphan just like me, please give me a boon for my protection. Rama, who is in the clan of kakutstha, the enlightened, the righteous and one who blooms righteousness, replied back that had hope and happiness in Sugriva that the fruit of helping is a friend and the bad doer is an enemy. Hence I'll kill him who has kidnapped your wife.

The above verses are available in the 8th sarga of the Kishkindakanda in Maharshi Valmiki's Ramayana. Sugreeva after understanding the power of Rama, begs him to help him by mentioning how dangerous his brother is and how he unrightfully has taken away everything from him (including his wife). Rama after listening to him mentions about who is a good friend and what constitutes an enemy and having said that he states that kidnapping one's wife is a cruel offense and an unrightful act and hence he will kill Vali.

What did Vishnu decide, listening to Gods?

इत्यतद्वचनम् श्रुत्वा सुराणां विष्णुरात्मवान्
पितरं रोचयामास तदा दशरथं नृपम्  

ityatadvacanam SrutvA surANAM viShNurAtmavAn

pitaraM rOcayAmAsa tadA daSarathaM nRupam

Shloka Translation:

Listening to the great words of the Gods, the great soul Vishnu
Decided that he will consider King Dasharatha as his father

Lord Vishnu decides on where he is to be born and who upon the request of the Gods, he had to choose as his father (Dasharatha). This shloka is available in the 15th Sarga of Balakanda within Maharshi Valmiki's Ramayana. Vishnu is the Lord himself who decides to be born after listening to the Gods about Ravana, who is in the world with his bad ruling and not being killed by anyone and since Dasharatha (who is a learned and a great king) was conducting a Yagna in anticipation of a Son, they all request him to provide a boon by dividing his divine soul into four and appear as four sons to Dasharatha's three wives. The next would be as to in what form Vishnu assures and blesses the Gods that he would fulfill their request.