The spell "Locate City" has a range of 10 miles per caster level, and allows the sense of distance and direction to the "nearest community of a minimum size designated by you at time of casting." With nearest being counted without movement through solid objects.
Thus, with a sufficiently large caster level, a sufficiently accurate census, and a sufficiently large population density of villages and cities, it should be quite possible to get a very good idea of where a person is based on gated counts of censuses.
The trick to building a navigation network out of this is by creating artificial communities in "known positions" which count for purposes of this spell. (How to do that is functionally a DM call, but a metropolis built in great miniature with undead squirrels haunt-shifted into it to count as "occupants" suspended in the sky isn't a... horrible option.) For each of these metropoli, anchor to them to the sky via 3 immovable rods within a spacing dedicated by the modal range of the spell being used (which itself becomes a question of assets and magic item economies.)
Much like with dGPS, since you can set the community size of each of these, it turns into a 4-colour map problem equivalent. By knowing north, having a chart of these undead beacons, and casting the spell N times (where N is the set of granularities allowed for by "minimum population" in the spell), you can get a direction and distance to each of the nearest "population-unique" beacons. Then it's a matter of triangulating against the map to identify where the individual is.
Circle dance would be preferable (as it has no maximum range), save for the fact that it requires firsthand knowledge of a creature, which rather reduces the utility, (though being taken on a tour of the "sky thrones" and meeting the undead squirrels sitting on each would itself, be hilarious). If you're OK with houserules, you can probably assert that each squirrel's truename can count as firsthand knowledge of them for purposes of circle dance. Then it's merely a matter of referencing the appropriate truenames of the beacons, and finding where the vectors intersect. But that's much less cool.
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Or they can just cast find the path (or Knowledge(Geography) perhaps boosted to absurd levels with divine insight or improvisation) to get there and if visibility prevents landing due to some obscure house-rule, use dispel fog to clear a lane to your landing area and blindsight and/or true seeing and/or short range teleportation and/or feather fall to deal with any traps or concerns at the landing pad.
The step from Spawn to Vampire is described on page 295 of the Monster Manual as:
If a true vampire allows a spawn to draw blood from its own body, the spawn transforms into a true vampire no longer under its master's control.
Interestingly enough, that suggests that any true Vampire can promote a Spawn, not just the Vampire that created the Spawn.
Best Answer
I think the most important thing to consider is just how "mobile" mobile is. Sure you can pull a Vandread and lug a coffin with you, but then you have a coffin that can surely be whittled away it in less than an hour to get to your defenseless body. So while a coffin on the move is possible due to the unstated RAW, where it is when you go back to it is vital. Just how many places can you trust to see your wispy form go flying by and not capitalize on the opportunityto kill or imprison you? So while mechanically this sounds like a great advantage (I'll move my coffin every four days between seven castles), who can you trust with your life to do this?
Additionally, almost every D&D campaign I've been in has taken some liberties on the more classical lore. Your DM (assuming you're not it) might have different ideas as to just what a vampire's weaknesses and abilities should be.