I agree with Lucius. It would be Muto, and hence magical, temporary, and dispellable. Hermatic magic can't really mesh the qualities of items like that.
You could do a Rego effect to blend two items physically. But that's different.
Muto is definitely the way to go, but I think the mistake is in concentrating on it merging 2 items. The correct way to look at it is that you want to take the strong sturdy vase and 'mutate' it into something that looks as good as the ornate vase. The fact that the 2nd vase is used in the spell is a component. Ie: +3 maybe to casting the Sturdy Vase into Ornate Vase if you have an Ornate Vase nearby to copy.
So, without a doubt, Muto is the answer, you just have to think of it as a component piece, not a merged piece, and have a description in the spell that says that if an Ornate vase is used in the casting of the spell, it disappears until the spell is either dispelled, or the vase is broken, etc.
My initial thoughts run along the same lines as Sphinx's. But I see rules issues cropping up.
You could develop a Muto XX spell that takes the qualities of object A and adds them to object B. However, you run into the problem that ,using the muto rules, adding a quality to object B with no refrence to any other object at all is really not so difficult. So why is a magus going to bother combining things? (perhaps a magical focus in combinations?).
I don't have the guidelines here but for a first -no reference to the rules- guess at using an object to stal a quality rqather than just creating it out of thin air, a one magnetude discount might be appropriate with the addition that the objects would be arcane conections to one another during the duration of the spell and by disturbing the "source" object the spell could be ended.
Of course this doesn't take two objects and make them into one composite object like you originally asked about.
This concept doesn't fit easily into the established system. You could create some new muto guidelines but as I start to think on the subject lots of exceptional cases and sticky issues come to mind (what if an item is enchanted?, what happens to the minds of merged people/animals? If flaws are carried over by muto transformations does a combined entity get both sets of flaws or neither or what?, etc.). That's alot to wade through. I don't think that I'd care to do the workunless the payoff was going to be really cool.
So, I don't think that this falls outside the bounds of hermetic magic. A magus should be able to create these sorts of spells without doing any crazy mystery initiation or fundimental research. The person in charge of the rules for the saga has a good bit of work in front of him, creating example spells testing how they'll work and looking for loopholes so that horribly exploitable or pathetically inefficient guidelines are not introduced.
This is what I'm really looking for. Let me give you an exemple. Take a bird & a lion to create a Sphinx like creature. I believe this is an interesting effect. My magus is looking into the essential nature boundary and these sort of spells always try to push the limit.
Another use would be to fuse the spirit of a man with another one. By spirit, I really mean spirit, not soul. Hence the men's soul would go on but his memories, personality & knowledge would survive into the new body. A bit like te ReMe that switches the mind but could actually be perceived as a gift by the receiving target
Living things are a completely different issue I believe. The 2nd Law of Magic is that you can not ever change a thing's essential nature. That means you can't make two things one. You could use a Bird to get a bonus to your casting roll for adding wings to a Lion, but you could not merge them because you're de-creating a thing's essential nature, either the Bird or the Lion.
Not really... The limit of essential nature simply says that magic needs to sustain the change in order for the change to take place & be maintained. Basically it means that you cannot use the Momentary duration for effects that fall into the essential nature category.
It says that if you violate a thing's essential nature, you have to maintain it because it tries to return to how it was. Then immediately afterwards says you can't change the essential nature of the thing itself. So, whereas you could turn a 3-legged dog into a 3-legged lion, you couldn't turn a 3-legged dog into a 4 legged lion. And the 3-legged lion would only be that way until you stopped concentrating, so no taking a duration of Sun or Diameter, etc for the spell.
More importantly it follows that by saying that you can change the way something appears, but can not effect what a thing is. Thus a vase will always be a vase. To use magic to make a vase not exist anymore (which is what you must do to turn 2 vases into 1) is a violation of that law.