It wouldn't have to be an early offshoot in terms of settlement, because we're not talking about an early innovation: we're just talking about the conservative failure to adopt innovation.
The 'Western' and 'Eastern' vowel systems can largely be explained as the result of only three sound changes:
a) raising of /e:/ and /i:/ relative to /i/, which spread rapidly through the urban Empire reaching almost everywhere eventually, but not Sardinia
b) analogous behaviour among the high back vowels, which likewise spread through the urban Empire, but less completely, notably failing to reach the Danubian provinces
c) loss of vowel length everywhere
[alternatively, you could explain the patterns by saying that c) was a constant pressure that took effect first in Sardinia and then in Danubia, thus making a) and/or b) impossible in those areas; I think this is a more elegant solution, but probably less likely, because it suggests sound changes happening first in the most backward areas. It's not impossible, though, especially given how many non-native speakers there were.]
[obviously there are complications, around umlaut and the treatment of nasal vowels, but as a simplification...]
To end up with a Sardinian system, you just have to avoid a) and b). That needn't be about how early your area is settled, because a) and b) spread much later than settlement in most areas anyway - they're part of the Imperial urban lingua franca, not an inheritence from initial colonisation. [another way of saying this: the Romance languages are better understood as the breakup of a common language into dialects through waves spreading though the language, rather than through a simple expansionary tree structure]
Instead, it's about how connected your language is to the late imperial world. I would make three suggestions:
a) you're more likely to be conservative in a rural area than in one with major entrepots. Hence Sardinia is actually very close to Rome, but being a pointless backwater* it was able to be very conservative - I would imagine that to late imperial romans, Sardinian farmers sounded like inbred hicks. So find somewhere relatively unpopulated. On the other hand, it needs enough population to maintain its language...
b) you're more likely to be conservative in a peripheral, or better yet isolated, area than one that's central and well-connected.
c) a big factor in a lot of the post-empire was de-urbanisation. The collapse of the cities led to an exodus of city folk into the countryside. I haven't seen anyone theorise this explicitly, but I very strongly suspect that some of the relative uniformity of Romance comes from the artificial levelling of dialects throug deurbanisation: cosmopolitan urban Romans, speaking something closer to a late imperial lingua franca, spread out and replaced the various rural dialects that might have been spoken in backwaters around the empire. I suspect that this is another reason Sardinia is unique: not only were there not many urban sardinians to begin with, but they were never expelled en masse into rural sardinia (because there was no major barbarian invasion), so eventually rural sardinian became the only variety. How can you exploit this? Well, one way is to avoid cities. Another is to say there are some small towns in your area, but no major invasion, so no urban exodus, and over time the towns just dwindle and the rural dialects take over. Another is to say that you're close enough to somewhere else that the urban cosmopolitans went somewhere else instead of into the country - eg perhaps the merchants in the port left on ships leaving only the backward farmers from the surrounding area. Or there could be a very effective invasion, that just slaughters the city folk en masse but doesn't bother actually settling and replacing the local farmers.
Intuitively, you'd think Britain would be perfect for a conservative romlang. In reality, however, British latin was apparently very close to that of Gaul, because of the interdependence of the two areas.