MacBook – Two Monitors Setup On a MacBook Pro

I believe you would like to take a look at this Endgadget link that explains (even with a video) what you need.

If you have two Thunderbolt ports (only the 27inch iMac), then those can support two external displays (confirmed by Apple, you can google that).

(this has been also asked on SuperUser)

The mini DisplayPort to DVI adapter will work great, so your 27" monitor is in.

The VGA monitor is problematic. HDMI and DVI share almost identical digital video signals, so any HDMI to DVI adapter (like the included one) has few (if any) conversion chips in it and the output DVI port will be DVI-D (digital only). DVI to VGA adapters also don't have conversion chips because they use DVI-I or DVI-A ports (A is analog only and I id both analog and digital): they take the DVI analog signal and move it straight to the VGA port. Unfortunately, that means that you cannot go from HDMI to VGA via DVI. You'll need a more expensive converter like this or this.

tl;dr: You can't go from HDMI to VGA without a converter; see the links for examples. For the dual-link DVI you need, the adapter you mentioned that will use the Thunderbolt port will work excellently.