Thanks to vam1980 in some other forum post:
"The adverbs 'heel' and 'zeer' are pretty much interchangeble (maybe 'zeer' is slightly stronger)."
In this sentence zeer, heel and erg are all interchangeable.
- zeer interessant = erg interessant = heel interessant = very interesting
But take care, all three of these also have different meanings, these are not interchangeable:
- zeer = pain
- het doet zeer = it is painful (lit. it does pain)
- heel = whole
- Ik heel wonden = I heal wounds
- een heel bord = a whole plate
- erg = bad
- het is erg = it is bad
So when combining them it comes with limits and you have to take care which of the meanings is used in all cases, also heel can be used in front of erg (in the meaning of very), to make it stronger, zeer and erg cannot be put in front of one of the others in the meaning of very, but they can in their other meanings:
- heel erg = zeer erg = very bad
- heel erg [iets] = very [something] (zeer cannot replace heel here)
- heel erg interessant = very interesting
- het doet erg zeer = het doet veel zeer = it hurts a lot (lit. it does very hurt/pain)
- (a bit farfetched) het bord viel, maar het is nog zeer heel = het bord viel, maar het is nog (heel) erg heel = the plate fell, but it is still very whole (in other words: not broken)
Also see RigelKentian's post here.