I want to know the significance of using
Jai before the mantra.
E.g. When I chant:
Om Vishnu Devtaya Namah and when I chant
Jai Shri Vishnu Devtaya Namah am chanting 1 mantra more significant/powerful than other?
Om is usually the starting point of most mantras. The reason is given below.
The syllable Om is the root from which the Vedas have sprung. (Every rite, therefore, should commence with the utterance of that syllable of vast import). Of him who has uttered for him the syllables Om, Namas, Swaha, Svadha, and Vashat, and who has according to the extent of his ability, performed sacrifices, and other rites, there is no fear in respect of next life in all the three worlds.
Mahabharata Santi Parva Section CCLXVIII
A mantra starting with Jai (rare case) can also be chanted if given by a competent Guru.
I agree with what Pradip Gangopadhyay stated in his answer that Aum is the starting point of all mantras as has been pointed out in the purport of the following verse SB 6.5.26:
At Nārāyaṇa-saras, the second group of sons performed penances in the same way as the first. They bathed in the holy water, and by its touch all the dirty material desires in their hearts were cleansed away. They murmured mantras beginning with oṁkāra and underwent a severe course of austerities.
Every Vedic mantra is called brahma because each mantra is preceded by the brahmākṣara (aum or oṁkāra). For example, oṁ namo bhagavate vāsudevāya. Lord Kṛṣṇa says in Bhagavad-gītā (7.8), praṇavaḥ sarva-vedeṣu: “In all the Vedic mantras, I am represented by praṇava, or oṁkāra.” Thus chanting of the Vedic mantras beginning with oṁkāra is directly chanting of Kṛṣṇa’s name.
Though I don't think Jai is proceeded usually in mantras, it is more of a salutation for instance Jai Mata Di or Jai Shri Ram.