Many claims and counter claims arise in this regard.Some claim its jain.Some claim its buddhist.SOmebody even claim it was christian.If we analyze thirukural we can find that it echoes vedas word by word.Not a single kural is against vedas.In fact we can say that thirukural is a tamil translation of the sanskrit vedas.
It starts with "Mathrudevo bhava".Then it goes on to "pithru devo bhava".
"Adhi,bhagavan muthatre ulagu......".No religion except hinduism worships mother and father.So in the first kural itself valluvar shows that kural is vedic based.
He calls bhramins as "arutholilor"(people with 6 duties).He says they will forget vedas,if king doesnt protect them.Sanskrit calls bhramins as "shat karma nirathar" meaning people with 6 duties.
He mentiones various vedic gods."indirane salung kari..." and "thamarai kannan ulagu...".He mentions indra and vishnu.He mentions pancha yagnas in vedas as "thenpulathar deivam virunthokkal than endrangu impulathar ombal thalai..."
And further he even mentions vedic principle of truth.Vedas defined truth as "sathyam buthahitham priyam..." meaning "words that cause benefit to society are truth...".Valluvar exactly said this principle as "vaimai enapaduvathu yaathenil yaathondrum theemai ilatha solal..."
And he mentions padivratha dharma too.Vedas say that a padivratha need not worship any god or do anything except praying to her husband.Valluvar says this as "deivam thola al kolunan than tholuvaal peiyena peyum malai..."
And yagnas are unique to only hinduism.He sings about them too.So to sum up he talks of so many things unique to hinduism like indra,vishnu,vaikunda,(thamarai kannan ulagu),bhramins,pathi vratha dharma,worshiping parents,pancha maha yagna etc.And word by word he echoes vedas.So we can say that thirukural is a complete vedic book.