It is a good idea and it will definitely make players happy. I have about 500,000 iso8 currently and this would be good.
However, from a business point of view:
Currently, the value or price (in USD) rounded off of each iso8 tier is as follow :
1,200 = $2
3,200 = $5
13,200 = $20
37,500 = $50
78,000 = $100
VIP = $10
Using the maximum of 1 million iso8 deposit as an example, it will net you extra 50,000(non-vip) or 100,000(vip) iso8 after 28 days.
50,000 iso8 is worth about $70, while 100,000 iso8 is worth about $130.
By attracting player to join VIP so that they could gain an extra 5% of interest, D3 would stand to gain:
$10 - 130 USD = - $120 (a negative)
If we were to use 200,000 iso8 instead, you would gain an extra 10,000 iso (non-vip) or 20,000 iso8 (vip) after 28 days.
10,000 iso8 is worth an estimated $15, while 20,000 iso8 is worth about $30.
In this case, D3 stand to gain either - $15 or -$20 (30-20).
The only way this is workable is that you adjust the interest rate to a more realistic percentage, or unless D3 scrap the buying of iso8 using real currency.