ARINC assumes a coverage of about 245nm for an aircraft at FL300. Both Natal (NAT) and Fernando de Noronha (FEN) are ACARS RGS's, but I put NAT at 590nm and FEN at about 420nm.
It does look like AF447 would have passed through several VHF ACARS sites prior to being out of range. Still hoping someone with first hand knowledge can comment on whether the VHF to SATCOM ACARS switch happens automatically as mentioned in the manual, or if maybe on a flight on this route it just uses SATCOM the whole time.
RGS List:
RGS list
(Note: Not a complete list.)
Map:
http://datalink.sas.se/ground/images/sita_eumea.pdf
ALCS65, they went down 400 miles from Natal (which I'd half expect to be VHF ACARS equipped and 160 miles from Fernando de Noronha (which might be ACARS equipped.) They were pretty much out of VHF communications for quite a period of time. Off hand I cannot find a reference to either of them supporting ACARS. I'd estimate they were a good hour out from any meaningful VHF contact, maybe (much) more. I'd expect the cut over to satellite would happen at or some modest time after the switch from VHF flight control communications to HF.
Released documentation reports the last several position reports as well as the standard ACARS all, they say, through Inmarsat's AOW bird.