Actually, the Gur and Vistani are essentially the same race (as are the traveller peoples of Greyhawk, and all other settings).
This interpretation was popular in Fourth Edition, but is not supported by canon sources either from within the Forgotten Realms or Ravenloft products prior to that time. Canonically, the Vistani of Ravenloft do not remember where they come from, and many do not care to think about it, as the Land of Mists is their home now.
Meanwhile, the Gur do have an origin within the Forgotten Realms -- they are a nomadic peoples from the Western Heartlands who interacted with the Rashemi and interbred with them so that the two peoples are now 'kin' (see the wiki article linked above). While some Gur may have travelled into Ravenloft, there's no indication that the Gur were the original Vistani, or even significantly became Vistani (for much the same reason that the Gur of the Forgotten Realms did not become absorbed into the Rashemi).
The link between these two races is confirmed in respect to Sybil who is herself Gur, but who is treated as Vistani by the Vistani themselves in Ravenloft.
Sybil? The only reference to Sybil I can find in Forgotten Realms lore is the half-fiend yuan-ti who served as Sseth's avatar during the Time of Troubles.
Are you talking about the fortune-teller who travels with Fai Chen? As far as I'm aware there is no published information on either character, and thus neither has an influence on canon, to this point.
With all that said, a DM can certainly make up whatever story sounds good at her table without worrying too much about canon; the only issue becomes what happens when canonical information (such as an encounter in a module) comes out later that contradicts what happened at the DMs table. At some point, it simply becomes easier to accept the canon rather than trying to continue to spin it to match what happened at a long-ago table.
Sybil's gifts with the sight are enhanced in Ravenloft as well - making her as powerful as a regular Vistani seer. Inversely, this implies that Vistani who leave Ravenloft will find their powers diminished (but not removed completely)
In the absence of a canonical information source, it would make more sense to say that Sybil has been adopted by a Vistani vardo (likely Madame Eva's own Zarovan vardo), granting her the full abilities of a vistani seer.
It's not wholly clear whether the Vistani derive their powers from the Dark Powers, from the Mists (insofar as the Mists are seen as something independent of the Dark Powers and not as their instrument), or from their 'special arrangement' with Strahd von Zarovich. All three have been proposed, and even the Vistani themselves may not know.
The travelers have many names on many worlds, but they are all the same people. The best way of looking at them, is each of the distinct traveler groups are extended tribes. The Gur are the Faerunian travelers, while the Vistani are the ones found in Ravenloft. Trace their lineage back far enough and you will find their root ancestry is the same.
This is the problem with reviving beloved material for a new 'look' and making arbitrary changes -- part of the draw of a Ravenloft-themed season is the nostalgia value involved in allowing old Ravenloft fans to revisit the environment with its new coat of paint, but changes like 'oh, yeah, the Vistani aren't mysterious orphans from an unknown world anymore, they're children of every world!' is that it pisses off people who appreciated the original material.
I'm not saying every DM has to be an expert on 2nd-and-3rd edition Ravenloft lore in order to run Curse of Strahd, but a DM who notes that some of her players seem to be holding back winces when she describes certain parts of the lore would be well-advised to ask what the issue is and consider adapting her explanations to fit that lore, or run the risk of alienating the very players who would best evangelize her game.
As another example, Curse of Strahd contains a well-known Greyhawk character whose portrayal here, and possibly very existence here, will likely enrage hard-core Greyhawk 'fanboys'. It only serves as an additional caution to be very careful, as a DM or as a designer, how you incorporate your call-outs to appeal to nostalgic fans, because you may end up offending the very people you're trying to delight.
--
Pauper