When vat deducted then is it necessary to deduct tds also??

TDS will be deducted for the labour/service portion not for the material portion....whatever the vat rate may be. 
If Invoice clearly says that bill amount is for the purpose of materials no TDS will be deducted.

Your question is not clear.

In purchases VAT is added and not deducted.

There is no TDS on purchase of goods. This is subject to clarification on your question.