Who are you buying the van from? Are they VAT-registered because if they are not then there won't be any VAT to claim back anyway.
You will not reduce your tax (I presume you mean Income Tax) liability all that much. The cost of the new van won't be allowed as a tax-deductible expense in just one year. You claim 12.5%% of the cost of the van as a capital allowance every year for the next 8 years.
eg - cost of van = €4,800 so you can claim €600 as a Capital Allowance every year for 8 years
Assuming tax rate is say 50% then you save €300 per year over the 8 years
Therefore total tax relief = €2,400
You will actually save more tax by claiming the VAT back and get Capital Allowances on the net-of-vat cost of the van anyway
eg - cost of van = €4,800 (incl VAT of €888). Therefore net cost of van = €3,902 so you can claim €488 as a Capital Allowance every year for 8 years
Again assuming tax rate is say 50% then you save €244 per year over the 8 years
So total Income Tax relief = €1,952 but you also get back the VAT of €888 (€4,800 / 1.23 * 0.23) so you actually gain €2,840 in this example
As far as when you have to register, per Revenue website: "registration is obligatory where certain turnover thresholds are exceeded or are likely to be exceeded in any twelve-month period"
Basically what that means is that you are obliged to register for VAT when you know that you will go over the threshold in any rolling 12-month period, not just necessarily your accounting year or calender year.