Here is an information from the BIR website. IMO, you should already register as a VAT registered taxpayer at the BIR. The threshold of 1.5M is for a year and not for a quarter. So if you already earn 620T in the first quarter, it is already a reasonable ground that you will have a more than 1.5M sales for this year.

From the BIR website:
Who are liable to register as VAT taxpayers?
Any person who, in the course of trade or business, sells, barters or exchanges goods or properties or engages in the sale or exchange of services shall be liable to register if:

1. His gross sales or receipts for the past twelve (12) months, other than those that are exempt under Section 109 (A) to (U), have exceeded One Million Five Hundred Thousand Pesos (P1,500,000.00): or
2. There are reasonable grounds to believe that his gross sales or receipts for the next twelve (12) months, other than those that are exempt under Section 109 (A) to (U), will exceed One Million Five Hundred Thousand Pesos (P1,500,000.00).

What is the liability of a taxpayer becoming liable to VAT and did not register as such?

Any person who becomes liable to VAT and fails to register as such shall be liable to pay the output tax as if he is a VAT-registered person, but without the benefit of input tax credits for the period in which he was not properly registered.