Couple other things for you:
Be aware of the file sizes of the objects that you place. Some are much larger than others and it might not be worth it to waste the file space over the single item that takes up 8-10% of storage.
Once you have placed an item, you can use identical version without taking as big a hit to your storage space. ie. You can place a single item 20-30 times without taking a big hit. But if you place 5 or 6 different items, you might use up too much space.
The Auto-snap feature only works on like items. The feature will not work on different objects. i.e. If you select a certain type of walkway, the snap feature only works if the objects are identical in all ways. You will need to have patience and zoom in to align unlike objects. Still do it. This makes for more unique stadiums.
Pay attention to what will be visible during actual gameplay. The cameras pan though the crowd or at the scoreboard between innings and plays. Those moments look best when you have built something that looks cool. While building a little city and adding other things around it is also cool, none of that really shows up during gameplay. Do something different that looks cool and actually impacts gameplay in a good way.
Fences. This is another touchy spot. I suggest determining what your outfield dimensions will be first thing when creating a new stadium. Pick the dimensions and layout, and build around it. There are pre-designed dimensions that are acceptable for online gameplay. You do not want a little bat with a redline appearing to indicate the field can not be used for competitive play. Very important- Save often. One quirk is that you can sometimes click on your fences and accidentally move them in or out, or make them taller or shorter. As soon as you make ANY change to the presets, the field can no longer be used online. You will never be able to manually move the fence back and make it ok. Your option will be to reset the fence dimensions, or quit at the moment and open your last saved file. If you have done any other customizations (like color, or adding windows), you don't want to have to reset your fences, and closing out to start over becomes the best option). So, don't mess with the fences and be prepared to accidentally move it sometimes and get frustrated when you can't put it back.
While this mode of the game is awesome, The Vault itself is a complete mess. There's still a lot of quirks and bugs. I have high expectations that the issues with the vault will be addressed next year. It is very frustrating sometimes to try to search or actually do anything inside the vault. So, you are not crazy. The functionality inside the vault really is broken.
If you want to see what your stadium looks like before saving it to the vault, you can play Homerun Derby or a game against the computer and be able to select a stadium not yet saved to the vault. You can select your unfinished stadium and play the game or Homerun Derby in it to get a feeling for how it looks and any modifications you want to make.
Have some fun. Be creative. Build some cool stuff. Before long you will be the one giving advice on how to use it.
